Fix coreboot revision detection

Fix coreboot revision detection

Neither do we publish coreboot via svn, nor is git-svn a useful indicator
anymore.  Instead, fetch a shortened commit id.

Change-Id: I1b990384553209a7d39ecf7f5e8a2db7c7e34d0b
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/110
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marcj303@gmail.com>