uboot-envtools: add env settings for ubnt,unifi-6-lr-ubootmod

Add settings for fw_printenv/fw_setenv for the Ubiquiti UniFi 6 LR
when running OpenWrt's version of U-Boot. The settings should work
equally with the unmodified version, but that has not yet been
tested.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>