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>

Makefile.inc

@@ -14,7 +14,8 @@
 ##
 
 ifneq ($(NOCOMPILE),1)
-GIT:=$(shell [ -e "$(top)/.git" ] && command -v git)
+GIT:=$(shell git -C "$(top)" rev-parse --git-dir 1>/dev/null 2>&1 \
+	&& command -v git)
 else
 GIT:=
 endif