octeon: add support for Ubiquiti UniFi Security Gateway

Ubiquiti UniFi Security Gateway (USG) is largely identical to
the EdgeRouter Lite (ERLite-3) apart from a different board ID
and two dome leds.

Device data (from WikiDev):
  CPU: Cavium Octeon Plus CN5020 @500MHz 2-cores
  Ethernet: 3x Atheros AR8035-A GbE PHY's
  Flash: On-board 4MB Flash
  Storage: Internal 3.8GB USB Flash (Kingston ID) drive
    w/ 1.5GB free for use occupies single internal USB port.
  Serial: 1x RJ45 port on front panel. 115200, 8N1
  Buttons: 1x Reset

Flash instructions are identical to EdgeRouter Lite.

Signed-off-by: Clemens Hopfer <openwrt@wireloss.net>

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

@@ -27,7 +27,8 @@ platform_copy_config_helper() {
 
 platform_copy_config() {
 	case "$(board_name)" in
-	erlite)
+	erlite|\
+	ubnt,usg)
 		platform_copy_config_helper /dev/sda1
 		;;
 	itus,shield-router)
@@ -91,7 +92,8 @@ platform_do_upgrade() {
 	ubnt,edgerouter-6p)
 		kernel=mmcblk0p1
 		;;
-	erlite)
+	erlite|\
+	ubnt,usg)
 		kernel=sda1
 		;;
 	itus,shield-router)
@@ -119,7 +121,8 @@ platform_check_image() {
 	erlite | \
 	itus,shield-router | \
 	ubnt,edgerouter-4 | \
-	ubnt,edgerouter-6p)
+	ubnt,edgerouter-6p | \
+	ubnt,usg)
 		local kernel_length=$(tar xf $tar_file $board_dir/kernel -O | wc -c 2> /dev/null)
 		local rootfs_length=$(tar xf $tar_file $board_dir/root -O | wc -c 2> /dev/null)
 		[ "$kernel_length" = 0 -o "$rootfs_length" = 0 ] && {