use BUSYBOX_STATIC when ADK_STATIC is choosen

package/busybox/config/Config.in

@@ -454,6 +454,7 @@ menu 'Build Options'
 
 config BUSYBOX_STATIC
 	bool "Build BusyBox as a static binary (no shared libs)"
+	default y if ADK_STATIC
 	default n
 	help
 	  If you want to build a static BusyBox binary, which does not