ar71xx: Add support for Teltonika RUT900

Teltonika RUT900 is a Router with LTE dual SIM, WiFi, 4x Ethernet
ports, I/O, RS232, RS485, GPS.

The device ist based on a Atheros AR9344 rev 3,

Specifications:
- 560/450/225 MHz (CPU/DDR/AHB)
- 128 MB of RAM
- 16 MB of FLASH
- Serial Console header on a Card Board edge connector
- 4x 10/100 Mbps Ethernet (3x LAN, 1x WAN)
- 2.4 GHz Wifi
- 2x external, detachable Wifi antennas
- LTE Modem Huawei ME909u-521 (Also other Modem seen)
- 2x LTE antennas
- 1x GPS antenna
- 7x LED, 1x button
- 1x USB Connector
- 1x Serial RS232
- 1x Serial RS485
- 1x MicroSD Card

The GPL sources of the device are available at www.teltonika.lt/gpl/
and are based on OpenWRT Barrier Breaker (14.07)

Running from tftp:

The Router starts into the uboot Webupdater if the Button ist pressed
more than 3 seconds, if no Network cable is attached it starts the
uboot serial console, from there the router loads the firmware image
via tftpboot from 192.168.1.2:firmware.bin (the router has the
192.168.1.1). With bootm the loaded image will be booted.

Signed-off-by: Steffen Weinreich <steve@weinreich.org>

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

@@ -1154,6 +1154,9 @@ ar71xx_board_detect() {
 	*"TL-WR941ND")
 		name="tl-wr941nd"
 		;;
+	*"TLT-RUT900")
+		name="tlt-rut900"
+		;;
 	*"Tube2H")
 		name="tube2h"
 		;;

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

@@ -443,7 +443,8 @@ platform_check_image() {
 	tl-wr941nd-v5|\
 	tl-wr941nd-v6|\
 	tl-wr940n-v4|\
-	tl-wr941nd)
+	tl-wr941nd|\
+	tlt-rut900)
 		local magic_ver="0100"
 
 		case "$board" in