riotboot_hdr enables to partition the internal flash memory
into "slots", each one with a header providing information
about the partition. The concept for now is limited to
firmware partitions, which are recognised by the riotboot
bootloader. In the future the concept might be extended to
represent other content.

Co-authored-by: Kaspar Schleiser <kaspar@schleiser.de>

Get BINFILE and ELFFILE from command line instead of environment variable.

Rename 'HEXFILE' to 'BINFILE' in the script as the binary file is used.
The documentation was already talking about 'BINFILE' but 'BINFILE'
was never exported by the build system and it was using 'HEXFILE' in the
implementation.

USEMODULE += crypto_aes_precalculated enables the precalculated T tables (the old
code).
USEMODULE += crypto_aes_unroll causes loops to be unrolled.

This makes doing 'scan-build-analyze' produce an error at execution if
WERROR=1.

When used from `scan-build` it will not procude an error to display the result
webpage.

This correctly defines a `scan-build-analyze` target that does not display the
result webpage.

`scan-build-view` has now been moved to a private target as should not
be used directly.

The handling of displaying the page on the host system and implementing
'scan-build-analyze' are now explicitely done with separate targets and
not double implemented target when in docker or on the host that were executed
twice with different implementations.

Update the SCANBUILD_OUTPUTDIR to be per board.
Also remove un-necessary export and 'CURDIR'.

This is required to have reference builds context using the right
RIOT_VERSION_OVERRIDE=buildtest.

When building from a worktree, the common git directory was not mounted
in docker.

This lead to the version not being set and issues with git-cache in
ubuntu bionic that could not execute the 'git hash-object' command.

tests/driver_ds18: Add test application for DS18B20 sensor.

tests/driver_ds18: Add whitelist of boards

`clean-intermediates` should be done per board. Without this it only
tries to clean the default board.

If GIT_CACHE_DIR is a directory make it available to docker.
This will allow using the system git_cache also in the docker container.

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

This completely removes the hardcoded/os specific path setting for includes.
It directly queries gcc include directories.

It takes what was already done by `makefiles/libc/llvm.inc.mk`.

I replaced the `GCC_MULTI_DIR` handling by givining `CFLAGS_CPU` when
searching for include directories.
`CFLAGS` cannot be used as it will crash when `-target $(CPU_ARCH)` is
added.
It currently requires using deferred variables as `CFLAGS_CPU` can be
overwritten later in the build process.

It is used by Makefile.base so should be exported globally even if only
defined by `llvm.inc.mk` for the moment.

Fixes #7707
Fixes #7800

Handle that `IMAGE_OFFSET` can be set to something that contain spaces.

With 'IMAGE_OFFSET=$$((0x1000 + 0x1000))' we had in the command line when doing
'flash'

    --offset $((0x1000 --offset + --offset 0x1000))

With the change we correctly have

    --offset $((0x1000 + 0x1000))

The variable was not given to docker before and as it is setting
'RIOT_VERSION_OVERRIDE=buildtest' it helps testing in docker more
consistently.

This helper Makefile is supposed to unify finding folders with
applications that can be built.

There are two major reasons for this:

1. clang picks up different errors sometimes than GCC.
2. OSX support is hardened as it is usually the toolchain used there.

Allow flashing with an offset in ROM from the rom base address.
It reuses `IMAGE_OFFSET` configuration variable name from `openocd.sh`.

This will allow flashing multiple images with different flash operations.

This mimics openocd behaviour that only erase needed sectors of the rom.

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

    While info-build already provides lots of useful information
    it does not print the application source dir. However this
    might be useful for debugging and logging, hence its added
    to the output.

When running tests, I often needed to modify `buildtests.inc.mk` to see the
actual errors/standard output.
This allows overwriting the default redirection of both to `/dev/null`.
It is low level and directly given to the command execution.

As it is interpreted by make, it can even be overwritten using build system
variables:

    'BUILDTEST_MAKE_REDIRECT=>/tmp/buildtest.$${board}.out 2>&1'