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

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

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

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.

Replacing usage will come in further commits/PRs.

Set the application module name with APPLICATION_MODULE.
The default module name is "application_$(APPLICATION)".

This should fix name conflicts where an application/test has the same name as
an existing module and so both write to the same archive.

Despite its name, UNDEF are files that must be included during linking even if
the linker would not by default.

Can be overridden from the environment by setting WERROR=0

also remove reversed logic for Werror in Makefile.buildtest
(a typo caused Werror to be always enabled for all builds)

export BUILD_IN_DOCKER=1 to use Docker for building inside a Docker
container.
GDB debug file paths will be wrong after building inside the container,
use the script provided in `dist/gdbinit-docker` to set the correct GDB
substitution paths (copy and paste the contents into your `~/.gdbinit`)

The environment variable DOCKER_FLAGS can be used to pass extra
parameters to the Docker process.

 - Makefile.base: Respect ARFLAGS when building static archives.
 - Makefile.cflags: Add default ARFLAGS.
 - Makefile.vars: Add description for ARFLAGS.

With many open PRs that could benefit from loading SDKs when needed,
instead adding vast amounts of code to RIOTs master, this PR provides
the "functions" `$(DOWNLOAD_TO_STDOUT)`, `$(DOWNLOAD_TO_FILE)`, and
`$(UNZIP_HERE)`.

The first "function" takes one argument, the URL from where to download
the content. It is then piped to stdout. To be used e.g. with `tar xz`.

The second "function" taken two arguments, the destination file name,
and the source URL. If the previous invocation was interrupted, then the
download gets continued, if possible.

The last "function" takes one argument, the source ZIP file. The file
gets extracted into the cwd, so best use this "function" with
`cd $(SOME_WHERE) &&`.

The clumsy name `$(UNZIP_HERE)` is taken because the program "unzip"
takes the environment variable `UNZIP` as the source file, even if
another file name was given on the command line. The rationale for that
is that the hackers of "unzip" hate their users. Also they sacrifice
hamsters to Satan.

- added CPU_MODEL variable
- added TERMFLAGS variable
- fixed a little style (dots on the end of comments)

- added targets to Makefile.include
- added DEBUGSERVER and RESET vars to Makefile.vars
- added output of those to Makefile.buildtests