ar71xx: change u-boot-env to read-write for ZyXEL NBG6616

As the ath79 port of this device uses a combined kernel + root
partition the uboot bootcmd variable needs to be changed. As using
cli/luci is more convenient than opening up the case and using a uart
connection, lets unlock the uboot-env partition for write access.

Signed-off-by: Christoph Krapp <achterin@googlemail.com>
(cherry picked from commit 982c1f6e42c5b3e0f23eedd825a317a2872aa37b)

target/linux/ar71xx/image/generic.mk

@@ -1067,7 +1067,7 @@ define Device/NBG6616
   BOARDNAME := NBG6616
   KERNEL_SIZE := 2048k
   IMAGE_SIZE := 15323k
-  MTDPARTS := spi0.0:192k(u-boot)ro,64k(env)ro,64k(RFdata)ro,384k(zyxel_rfsd),384k(romd),64k(header),2048k(kernel),13184k(rootfs),15232k@0x120000(firmware)
+  MTDPARTS := spi0.0:192k(u-boot)ro,64k(env),64k(RFdata)ro,384k(zyxel_rfsd),384k(romd),64k(header),2048k(kernel),13184k(rootfs),15232k@0x120000(firmware)
   CMDLINE += mem=128M
   RAS_BOARD := NBG6616
   RAS_ROOTFS_SIZE := 14464k