ath79: align naming of Ubiquiti Nanostation M

ath79: align naming of Ubiquiti Nanostation M

Support for the Nanostation M (XW) was added in 40530c8eb with board
name "nanostation-m-xw". The current image for the "Nanostation M"
uses "nano-m" as the board name.
This commit renames it to the full product name as it's used by all
other boards. The legacy boardname of the ar71xx target is added
via SUPPORTED_DEVICES to ease switching to ath79 target.

Signed-off-by: Sven Roederer <devel-sven@geroedel.de>
[touch-ups on the commit message, removed subject remains]
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>