Using symlinks the submodules are added to appear just like any
downloaded unpacked tar.
Also added a Makefile clean rule to clean the submodules, which
includes Reseting the submodules to the origin branch state,
including ignored files.
This commits enables gdb's python support. In order to make it work, we
had to create a python fork with some patches to the buildsystem, and
also had to patch gdb as well.
