use empty default for keyboard layout, rebuild when changed

mk/build.mk

@@ -88,6 +88,10 @@ POSTCONFIG=		-@\
 			touch .rebuild.dropbear .rebuild.openssh;\
 			rebuild=1;\
 		fi; \
+		if [ "$$(grep ^ADK_RUNTIME_KBD_LAYOUT .config|md5sum)" != "$$(grep ^ADK_RUNTIME_KBD_LAYOUT .config.old|md5sum)" ];then \
+			touch .rebuild.bkeymaps;\
+			rebuild=1;\
+		fi; \
 		if [ $$rebuild -eq 1 ];then \
 			cp .config .config.old;\
 		fi; \