This allows setting other GLOBAL_GOALS by setting it before.
This could be the case for the application `Makefile` or files parsed
from `RIOT_MAKEFILES_GLOBAL_PRE`.

This introduces a new environment variable for a common directory
that holds all output of the build process, such as application or
package binaries. This would also allow to easily redirect output
to any other location, e.g. for out-of-source builds.

riotboot is introduced here and makes use of riotboot_hdr,
which indentifies the images encapsulated as slots.
The slot size and offset is configurable, which makes
slots extendable if needed, e.g. 2 or more slots can be
transparently added.

Co-authored-by: Kaspar Schleiser <kaspar@schleiser.de>
Co-authored-by: Gaëtan Harter <gaetan.harter@fu-berlin.de>

When flash-only was introduced (in #8373), the `flash` rule was
made conditionally dependent on `all` by looking for `flash-only`
in MAKECMDGOALS. This was done to avoid code duplication.

There's a cleaner way, by using canned recipes. When we upgrade the
requirements to gnu make 4, the flash recipe can be defined as ?=.

This rule is not being used, it complicates the makefile and causes
make clean to permform unnecessary actions.

All packages have a Makefile.include, so the rule is not needed anyways.
Also, it is defined with a double colon for no reason.

This allows specifying a list of files that should be parsed by make at
the beginning and at the end of Makefile.include.

It is a generic mechanism to allow specifying system wide configuration:

* Globally overwrite the 'TERMPROG'
* Specify a hard written port / debug_adapter_id for some BOARD values
* Define you own specific targets
* Override default targets

It can include file before and after Makefile.include to allow handling
different configurations.

When building in parallel, clean is done at the same time as the
container starts building.

So the parallel `clean all` handling should also be done for the
`..in-docker-container` target.

To keep Makefile.include clean and to be consistent with other tools,
the Eclipse IDE support is put in a separate file.

Makefile.include contained logic to clone a different RIOT version and
build the application against it. This can be easily achieved by making
a git worktree and overriding RIOTBASE, without adding any complexity
to our makefiles. Therefore, the feature is being removed.

This fixes building an example that is outside of RIOT with docker and
also fixes building from a release archive.

When an application only includes Makefile.include without specifying
RIOTBASE and uses a `Makefile.local` file, RIOTBASE would use a wrong
default value and get the value of the directory where `Makefile.local` is.

The hostname does not provide any additional or necessary information to
the RIOT_VERSION string. On the contrary, some might consider the hostname
as personal information, which should not be exposed unsolicitedly.

Currently the default value was set by `dist/tools/git/git-cache`.
This moves/duplicates the default value in the build system.

Process `Makefile.include` for external modules. It is included after the others
so it could overwrite some of the configuration if wanted.

Process `Makefile.dep` for external modules. It is included before the others so
it could be parsed before setting 'default' values to dependencies.

This prepares for being able to always include Makefile.features
globally. It prevents new board for missing this until it is done.

It fixes issues with the current rule that it is wrong that each
`$(BINDIR)/pkgname.a` is built by going in the package directory:

* `nordic_softdevice_ble.a` is built using `DIRS` and so the
  `$(APPLICATION_MODULE).a` target.
* It prevents having packages that are pseudomodules, which may be
  required to only use one "library" part of a package.

It also simplifies handling changes in 'ld -r' PR that could produce
objects instead of archives for packages.

Limitation of the current implementation
----------------------------------------

It removes rules being 'file' target and makes them depend on `.PHONY`
targets so always forces re-build.
But having a file target whose file is silently generated by another
target does not trigger a rebuild in Make.

They may have been declared as `order-only` prerequisites but as there are
some edge-cases that may not always work and does not currently add anything,
it was decided to keep them as normal prerequisites until it can be
globally solved.

BINDIR has already have been created by `pkg-prepare` and the real build
directory for the package will be created by the make target itself.

Explicit the target name so the stem can be extracted.

Now also print warnings if there are missing features or a blacklisted
board when doing 'make flash'

Before, only when doing 'make' or 'make all' the FEATURES_REQUIRED,
BOARD_BLACKLIST/WHITELIST and TOOLCHAINS were checked and a warning was printed.

However as 'flash' triggers 'all' it is a common case to do 'make flash'
directly instead of 'make all flash'. So better also print warnings in
this case.

makefiles/info-global.inc.mk already includes this file unconditionally.
So it is misleading to say it is optional.

Remove file management from `genconfigheader` script and use `lazysponge` in
Makefile.include.

Use --verbose option when in non QUIET building mode.

Assuming `TOOLCHAIN_SUPPORTED` is provided by the board and
`TOOLCHAIN_BLACKLIST` by a module or `pkg`, this outputs the fact that
a toolchain is not supported or blacklisted in a similar manner as
(un-)supported features and boards are.

When downloading the release archive and building an example, the RIOT_VERSION string is not set to "Version 2018.04" but to
    
    Version: UNKNOWN (builddir: /home/me/Downloads/RIOT-2018.04)

This allows sourcing a global VERSION file to manually set it before releases.

Introduce dist/pythonlibs directory to store RIOT python packages.
This directory is exported via PYTHONPATH by the build system to
make it commonly available.

The rule to rebuild the Makefile.include(s) should be removed at some
point.

When selecting a directory to build using `-C` with make
`BUILD_IN_DOCKER` will fail, because the `BUILDRELPATH` chooses the path
`make` is executed in, not the path selected by `-C`. This fixes this
bug by replacing `PWD` in the macro's definition with `CURDIR`.

Remove debug '$(info)' added by https://github.com/RIOT-OS/RIOT/pull/9451

    Fixes a regression introduces #9479 which breaks building on macOS.
    The introduces `uniq` function is never called, hence the includes
    ie. USEMODULE_INCLUDES are not used.

Currently, `USEMODULE_INCLUDES` is deduplicated using a combination of
`tr` and `awk`. The proposed method achieves a deduplication (no
sorting) without the need to shell out by using make internals only.

Use BUILDDEPS to define pkg-prepare dependency

Use BUILDDEPS to define the APPDEPS dependency