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>

SVN-Revision: 41930

target/linux/ar71xx/nand/target.mk

 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