The ar71xx images for the Ubiquiti NanoStation M (XM) devices use
"nanostation-m" as board name, but the ath79 images are only
compatible with the "nano-m" board name, so sysupgrade complains.
By changing this additional supported device, sysupgrade smoothly
upgrades from ar71xx to ath79.
Ref: openwrt#2418
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit f473ce6f23)
081d96bece