kbuild: tag with git revision when git describe is missing

setlocalversion used to use an abbreviated git commit sha1 to generate the
tag.  This was changed in commit d882421f
"kbuild: change CONFIG_LOCALVERSION_AUTO to use a git-describe-ish format"
to use git describe to come up with a tag.  Which is nice, but git describe
sometimes can't describe the revision.
Commit 56b2f070

 ("setlocalversion: do not
describe if there is nothing to describe") addressed this, but there is still
no tag generated.

So, generate a plain abbreviated sha1 tag like setlocalversion used to when
git describe comes up short.

Signed-off-by: Trent Piepho <tpiepho@freescale.com>
CC: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>