git tracker: add special version string to build

git tracker: add special version string to build

In output trees created with the git tracker, we avoid changing
the "versions" file all the time and just put "(see git)" into
the versions. However, it's useful to still be able to identify
it, and when using this the backport is (usually) built in the
git tree. Use this fact and add the git tree commit into a new
variable to put it into the MODULE_VERSION and also print it at
runtime.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>