ar71xx: add support for RB LHG 5nD

This patch adds support for the MikroTik RouterBOARD LHG 5
https://routerboard.com/RBLHG-5nD

Specifications:
- SoC: Atheros AR9344 (600MHz)
- RAM: 64MB
- Storage: 16MB NOR SPI flash
- Wireless: builtin AR9344 5GHz, 2x2:2
- Ethernet: 1x100M

Initial flash via initramfs, like all other RB devices.

Signed-off-by: Thibaut VARENE <hacks@slashdirt.org>

target/linux/ar71xx/base-files/lib/upgrade/platform.sh

@@ -638,6 +638,7 @@ platform_check_image() {
 	rb-750up-r2|\
 	rb-941-2nd|\
 	rb-951ui-2nd|\
+	rb-lhg-5nd|\
 	rb-mapl-2nd)
 		return 0
 		;;
@@ -689,6 +690,7 @@ platform_pre_upgrade() {
 	rb-750up-r2|\
 	rb-941-2nd|\
 	rb-951ui-2nd|\
+	rb-lhg-5nd|\
 	rb-mapl-2nd)
 		# erase firmware if booted from initramfs
 		[ -z "$(rootfs_type)" ] && mtd erase firmware