build system: Only setup git hooks if we're in a git checkout

A bit crude test, but before we would have _created_ .git
and confused later git presence tests.

Change-Id: Iec882d0e38ce1bd227cae8c1e541fb21be085290
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: http://review.coreboot.org/8601
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

Makefile.inc

@@ -436,6 +436,7 @@ lint lint-stable:
 	rm -f $$LINTLOG
 
 gitconfig:
+	[ -d .git ]
 	mkdir -p .git/hooks
 	for hook in commit-msg pre-commit ; do                       \
 		if [ util/gitconfig/$$hook -nt .git/hooks/$$hook -o  \