handle/make/file: check for all default makefiles

Previously, this script only checked for "Makefile",
but "makefile" is another valid name; additionally, if
GNU Make is used, "GNUmakefile" is an accepted default.

Signed-off-by: Leah Rowe <leah@libreboot.org>

script/handle/make/file

@@ -44,7 +44,8 @@ handle_dependencies()
 
 run_make_command()
 {
-	[ -f "${project}/Makefile" ] || return 0
+	[ -f "${project}/Makefile" ] || [ -f "${project}/makefile" ] || \
+	    [ -f "${project}/GNUmakefile" ] || return 0
 	if [ -z "${mode}" ]; then
 		x_ make -C "${project}" -j$(nproc)
 	else