make -f test-checks.mk test-ensure_value should succeed, while
make -f test-checks.mk test-ensure_value-negative should fail.

A call to `$(ensure_value x,y)` will fail with message y if x is empty, and
otherwise return x. This can be useto write more compact makefiles, while still
producing friendly error messages.

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

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'

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.