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'

Comments inside an if are usually also indented.

It replaces

    make BOARD=iotlab-m3 info-debug-variable-NEWLIB_INCLUDE_DIR
    /usr/bin/../lib/gcc/arm-none-eabi/7.2.1/../../../../arm-none-eabi/include/

with

    make BOARD=iotlab-m3 info-debug-variable-NEWLIB_INCLUDE_DIR
    /usr/arm-none-eabi/include

Without trailing slash and without relative '..' everywhere.

It also uses `realpath` instead of `abspath` to support Mingw32.

Some versions of Mingw32 abspath implementation has trouble working with
windows formatted path.

    $(abspath "C:/A/B") returns "/C/CUR/DIR/C:/A/B" instead of "/C/A/B"

relpath does not have this problem, it does additional symlink resolution but is
not a problem.
Note: on windows it does not remove the trailing `/`.

https://github.com/zephyrproject-rtos/zephyr/issues/2061#issuecomment-331635063

Patched in

https://github.com/zephyrproject-rtos/zephyr/commit/941059c69f9488613775a1a8802a099447cdea4c

Only keep lines that are indeed include path.
It also keeps newlines as they do not matter.

It fixes Mingw32 support where `grep '^\s'` is not working the same way.
It also handles some mac `sed` that do not support `\s`.

Ouput tested with:

    make -C examples/hello-world BOARD=samr21-xpro info-debug-variable-COMPILER_INCLUDE_PATHS
    # by also putting newlines for readability

Now:

    /usr/bin/../lib/gcc/arm-none-eabi/7.2.1/include
    /usr/bin/../lib/gcc/arm-none-eabi/7.2.1/include-fixed
    /usr/bin/../lib/gcc/arm-none-eabi/7.2.1/../../../../arm-none-eabi/include

Before:

    /usr/bin/../lib/gcc/arm-none-eabi/7.2.1/cc1 -E -quiet -v -iprefix /usr/bin/../lib/gcc/arm-none-eabi/7.2.1/ -isysroot /usr/bin/../arm-none-eabi -D__USES_INITFINI__ /dev/null
    /usr/bin/../lib/gcc/arm-none-eabi/7.2.1/include
    /usr/bin/../lib/gcc/arm-none-eabi/7.2.1/include-fixed
    /usr/bin/../lib/gcc/arm-none-eabi/7.2.1/../../../../arm-none-eabi/include

As NEWLIB_INCLUDE_DIR has already been set here, with an empty value, it is not
overwriting it because of the '?='.

When NEWLIB_INCLUDE_DIR is set from other parts than 'COMPILER_INCLUDE_PATHS' it
does not have a trailing slash.
Also, it makes it more problematic when supplying it from the command line.

And anyway having two '/' does not break anything.

Not all users register their user in the docker group and should not run the
whole make process as root to use docker.

They can now overwrite DOCKER with `DOCKER="sudo docker"`.

Creating files as a user is correctly handled as `id -u` is still run as the
original user.

In the previous state, with llvm and arm for example, newlib-nano include dir
NEWLIB_NANO_INCLUDE_DIR is placed after NEWLIB_INCLUDES and so the default
'newlib.h' is used instead of the nano version.

llvm is defining NM already and `openthread` is using the symbol.

gcc does an error but not clang, so add -Werror.

The excluded patterns can always be defined as they only set `CFLAGS` that
should not be passed to `CXX`.

This prepares for replacing the cflags support detection by a function.

WERROR already sets -Werror so this is not necessary.

BUILDDEPS are files / make targets that should be build before compiling.
It can include packages source download, generating headers, modules.

It is the equivalent of `APPDEPS` but not limited to the application.
It cannot be done right now with `APPDEPS` as it is used in `BASELIBS` and
fixing it requires changing mips using it for source files.

Officially allow packages to define pseudomodules in their Makefile.include as
done in the libcose package.

The sensor family SHT10, SHT11 and SHT15 only differ in their accuracy (as in
calibration, not as in resolution). Thus, the same driver can be used for all.
The new driver name better reflects this fact.

- introduce common place for boards using stlink: same serial, all use openocd
- apply this to nucleos

I'm using something like this command for a while now for debugging
GNRC. Usually, I just patch it into the application I'm using it with,
but I think there is a benefit to also provide it to RIOT upstream
properly.

Warn if a user-supplied include directory does not exist.

This reverts commit 88aaa623.

New CPU FE310 from SiFive based on RISC-V architecture

build: add makefile for RISC-V builds

Makefile for builds using RISC-V tools

The newlib-nano include directory can be named either newlib-nano or
nano, so we should make sure it is added in both cases.

Works around a regression in gcc 8.1.
See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85606.

Add a target to print an internal variable value.

Usage: make info-debug-variable-VARIABLENAME

Example:

    make info-debug-variable-ELFFILE
    /path/to/riot/examples/hello-world/bin/native/hello-world.elf

Replacing usage will come in further commits/PRs.