Both of if and else parts append exactly 12 hex chars, but in
different ways.

Factor out the else part because we need to support it without relying
on git-describe. Remove the --abbrev=12 option since we do not use the
hash from git-describe anyway.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Nico Schottelius <nico-linuxsetlocalversion@schottelius.org>