gitconfig: Use the right make executable in git hooks

When installing git hooks through $(MAKE) gitconfig,
make knows itself (and is a GNU make). So let it splice
itself into hooks where necessary by replacing %MAKE%.

Change-Id: Iaf778bfa3f17a8fe31312f871571ed89a9de5385
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/10018
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

util/gitconfig/pre-commit

@@ -1,2 +1,2 @@
 #!/bin/sh
-exec make lint-stable
+exec %MAKE% lint-stable