ar71xx: make target source-only

Lets make it really explicit, that we should now focus only on ath79 in
order to make it ready for next release, where ar71xx is going to be
removed for good.

Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/ar71xx/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 ARCH:=mips
 BOARD:=ar71xx
 BOARDNAME:=Atheros AR7xxx/AR9xxx
-FEATURES:=usbgadget
+FEATURES:=usbgadget source-only
 CPU_TYPE:=24kc
 SUBTARGETS:=generic tiny nand mikrotik