ath79: add SUPPORTED_DEVICES to ubnt_nanostation-m-xw

The ar71xx images for the Ubiquiti NanoStation M (XW) devices use
"nanostation-m-xw" as the board name, but the ath79 images are only
compatible with the "nano-m-xw" board name, so sysupgrade complains.

By adding this additional supported device, sysuspgrade smoothly
upgrades from ar71xx to ath79.

Tested on a NanoStation M (XW) running OpenWrt ar71xx r10250-016d1eb.

Ref: https://github.com/openwrt/openwrt/pull/2418
Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>
[removed duplicate DEVICE_VARIANT, removed uneeded nano-m-xw support]
Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit 6dda2ea6ad)
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

target/linux/ath79/image/generic-ubnt.mk

@@ -116,7 +116,7 @@ TARGET_DEVICES += ubnt_nanostation-m
 define Device/ubnt_nanostation-m-xw
   $(Device/ubnt-xw)
   DEVICE_TITLE := Ubiquiti Nanostation M (XW)
-  SUPPORTED_DEVICES += nano-m-xw
+  SUPPORTED_DEVICES += nanostation-m-xw
 endef
 TARGET_DEVICES += ubnt_nanostation-m-xw