Makefile.inc: Improve git worktree check

`Makefile.inc` checks for `.git` to be present under $(top) to define
the value of $GIT. This check is rather weak and doesn't handle many
edge cases like that of a broken gitfile.

Add a proper `git rev-parse` call to check the condition.

Change-Id: Ifd6da19f13d9f2a9fddb6afd7cb5f16daba2401e
Signed-off-by: Alex Thiessen <alex.thiessen.de+coreboot@gmail.com>
Reviewed-on: https://review.coreboot.org/23254
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
1 file changed