Add linux_menuconfig target

A handy shortcut to change the config of your kernel.

Change-Id: I1341d10a671222920b933a84e941dc42578537da
diff --git a/Makefile b/Makefile
index a7be173..8dbe270 100644
--- a/Makefile
+++ b/Makefile
@@ -207,6 +207,10 @@
 linux_defconfig $(LINUX_CONFIG):
 	+ $(LINUX_MAKE) $(LINUX_DEFCONFIG)
 
+.PHONY: linux_menuconfig
+linux_menuconfig: $(LINUX_CONFIG)
+	+ $(LINUX_MAKE) menuconfig
+
 .PHONY: linux
 linux: $(LINUX_CONFIG)
 	+ $(LINUX_MAKE)