ar71xx: add RTC feature flag to ar71xx/nand

without this the build for NBG6716 fails due to missing packages

https://dev.openwrt.org/ticket/17325

Signed-off-by: John Crispin <blogic@openwrt.org>

Backport of r41930

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@41971 3c298f89-4303-0410-b956-a3cf2f4a3e73

target/linux/ar71xx/nand/target.mk

@@ -1,5 +1,5 @@
 BOARDNAME := Generic devices with NAND flash
-FEATURES += squashfs nand
+FEATURES += squashfs nand rtc
 
 define Target/Description
 	Build firmware images for Atheros AR71xx/AR913x based boards with