uboot-envtools: ath79: add support for Ubiquiti XM devices

uboot-envtools: ath79: add support for Ubiquiti XM devices

Inspired by commit 9565c5726a34da7c9c953d2293b70fdbfef0e0be, and by
facts that all Ubiquiti XM devices share flash layout, and images are
mostly compatible between all of them - enable uboot-envtools support for
whole XM line.

Build tested on: Ubiquiti Airrouter, Bullet-M (7240,7241), Nanobridge-M,
Nanostation-M (+ Loco), Picostation-M, Powerbridge-M, Rocket-M.
Runtime tested on: Ubiquiti Nanobridge M5 (XM).

Signed-off-by: Lech Perczak <lech.perczak@gmail.com>