kernel: Hide kernel options behind a menu

We are starting to add more and more kernel configurable options, to the
point where the Global build options menu is not really usable anymore,
hide all kernel-related configuration options behind a menu.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

config/Config-build.in

@@ -83,10 +83,12 @@ menu "Global build settings"
 		  Useful for release builds, so that kernel issues can be debugged offline
 		  later.
 
-	comment "Kernel build options"
+	menu "Kernel build options"
 
 	source "config/Config-kernel.in"
 
+	endmenu
+
 	comment "Package build options"
 
 	config DEBUG