ramips/mt76x8: Add support for TP-Link TL-WR841N v13

Flashing instructions:

The image can only be flashed via TFTP, not via WebUI.

1. Configure PC with static IP 192.168.0.66/24 and tftp server.
2. Rename "...-tftp-recovery.bin" to "tp_recovery.bin" and place
   it in tftp server directory.
3. Connect PC with one of LAN ports, press the reset button, power up
   the router and keep button pressed for around 6-7 seconds, until
   device starts downloading the file.
4. Router will download file from server, write it to flash and reboot.

ref: openwrt/openwrt@24043a0

Signed-off-by: Dominik Heidler <dominik@heidler.eu>
Reviewed-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
[add commit message, rebase]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Fabian Bläse <fabian@blaese.de>

bsp/board_mt76x8.bsp

@@ -2,4 +2,5 @@ machine=mt76x8
 chipset=ramips
 subtarget=mt76x8
 images=("openwrt-${chipset}-${subtarget}-tplink_c50-v3-squashfs-*"
+        "openwrt-${chipset}-${subtarget}-tl-wr841n-v13-squashfs-*"
         )

bsp/mt76x8/.config

@@ -6,6 +6,8 @@ CONFIG_TARGET_ramips_mt76x8=y
 CONFIG_TARGET_MULTI_PROFILE=y
 CONFIG_TARGET_DEVICE_ramips_mt76x8_DEVICE_tplink_c50-v3=y
 CONFIG_TARGET_DEVICE_PACKAGES_ramips_mt76x8_DEVICE_tplink_c50-v3=""
+CONFIG_TARGET_DEVICE_ramips_mt76x8_DEVICE_tl-wr841n-v13=y
+CONFIG_TARGET_DEVICE_PACKAGES_ramips_mt76x8_DEVICE_tl-wr841n-v13=""
 CONFIG_BUSYBOX_CUSTOM=y
 CONFIG_TARGET_PER_DEVICE_ROOTFS=y
 # CONFIG_BUSYBOX_CONFIG_BRCTL is not set

src/packages/fff/fff-network/ramips/network.tl-wr841n-v13

@@ -0,0 +1,12 @@
+. /lib/functions/system.sh
+
+PORTORDER="0 1 2 3 4"
+
+WANDEV=eth0
+SWITCHDEV=eth0
+CLIENT_PORTS="6t 3 4"
+WAN_PORTS="6t 0"
+BATMAN_PORTS="6t 1 2"
+
+ROUTERMAC=$(cat /sys/class/net/eth0/address)
+ETHMESHMAC=$(macaddr_setbit_la "$ROUTERMAC")