busybox: Add busybox-xconfig and busybox-gconfig rules

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/busybox/busybox.mk

@@ -178,8 +178,9 @@ endef
 
 $(eval $(call GENTARGETS,package,busybox))
 
-busybox-menuconfig: busybox-patch
-	$(BUSYBOX_MAKE_ENV) $(MAKE) $(BUSYBOX_MAKE_OPTS) -C $(BUSYBOX_DIR) menuconfig
+busybox-menuconfig busybox-xconfig busybox-gconfig: busybox-patch
+	$(BUSYBOX_MAKE_ENV) $(MAKE) $(BUSYBOX_MAKE_OPTS) -C $(BUSYBOX_DIR) \
+		$(subst busybox-,,$@)
 	rm -f $(BUSYBOX_DIR)/.stamp_built
 	rm -f $(BUSYBOX_DIR)/.stamp_target_installed