ar71xx: add model detection for UniFi AC-LR

ar71xx: add model detection for UniFi AC-LR

This commit adds correct model detection for UniFi
AC-LR. Previously, said device was incorrectly detected
as UniFi-AC-LITE/MESH.

The Information about the device is stored at 0xC in the EEPROM
partition. It corresponds to the sysid in /etc/board.info of the
Ubiquiti stock firmware.

Signed-off-by: Andreas Ziegler <dev@andreas-ziegler.de>
[adjust naming style of target to existing ones]
Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit 2bc7c519dcc0e7ca09feed24a3b9a105bc49f423)
Signed-off-by: David Bauer <mail@david-bauer.net>