sdk: track files with git and use it to implement proper clean targets

sdk: track files with git and use it to implement proper clean targets

Initialize a Git repository in the SDK and use git reset / git clean
to rollback any SDK changes with "make clean" or "make dirclean".

This approach is more robust than nuking entire directory trees because
some parts of them might have been shipped with the original archive.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>
SVN-Revision: 43904