Add dirty flag to git describe

git describe knows --dirty, which adds -dirty to the verion number
if the tree contains uncommited changes. We should add this flag
to make it obvious that the COREBOOT_VERSION might be misleading.
This is especially important as this version number is now used
in the SMBIOS data structures.

Change-Id: If4c608c7455e1bbf0cc530c6299fa00eb0fe4d58
Signed-off-by: Sven Schnelle <svens@stackframe.org>
Reviewed-on: http://review.coreboot.org/173
Tested-by: build bot (Jenkins)