kbuild: Use wildcard for driver subdirectories

Change-Id: Id1685c0b28ec8e3ab972a671af6f2de6f321c645
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: http://review.coreboot.org/9805
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/drivers/uart/Makefile.inc

@@ -1,3 +1,5 @@
+ifeq ($(CONFIG_DRIVERS_UART),y)
+
 romstage-y += util.c
 ramstage-y += util.c
 bootblock-y += util.c
@@ -28,3 +30,5 @@ bootblock-$(CONFIG_BOOTBLOCK_CONSOLE) += pl011.c
 romstage-y += pl011.c
 ramstage-y += pl011.c
 endif
+
+endif