kbuild: fix scripts/setlocalversion with git

When using trees like wireless-testing, which have untagged tags,
scripts/setlocalversion does not display any git indication for
localversion.

This patch fixes it: If git is available, but no usable tag is found,
it uses -g${head}. It skips the detection of unanottated tags via
git name-rev.

Signed-off-by: Nico Schottelius <nico@ikn.schottelius.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>