ath79-generic: Add support for OpenMesh MR600 v1

Device specifications:
======================

* Qualcomm/Atheros AR9344 rev 2
* 560/450/225 MHz (CPU/DDR/AHB)
* 128 MB of RAM
* 16 MB of SPI NOR flash
  - 2x 7 MB available; but one of the 7 MB regions is the recovery image
* 2T2R 2.4 GHz Wi-Fi
* 2T2R 5 GHz Wi-Fi
* 4x GPIO-LEDs (2x wifi, 1x wps, 1x power)
* 1x GPIO-button (reset)
* TTL pins are on board (arrow points to VCC, then follows: GND, TX, RX)
* 1x ethernet
  - AR8035 ethernet PHY (RGMII)
  - 10/100/1000 Mbps Ethernet
  - 802.3af POE
  - used as LAN interface
* 12-24V 1A DC
* internal antennas

Flashing instructions:
======================

Various methods can be used to install the actual image on the flash.
Two easy ones are:

ap51-flash
----------

The tool ap51-flash (https://github.com/ap51-flash/ap51-flash) should be
used to transfer the image to the u-boot when the device boots up.

initramfs from TFTP
-------------------

The serial console must be used to access the u-boot shell during bootup.
It can then be used to first boot up the initramfs image from a TFTP server
(here with the IP 192.168.1.21):

    setenv serverip 192.168.1.21
    setenv ipaddr 192.168.1.1
    tftpboot 0c00000 <filename-of-initramfs-kernel>.bin && bootm $fileaddr

The actual sysupgrade image can then be transferred (on the LAN port) to the
device via

    scp <filename-of-squashfs-sysupgrade>.bin root@192.168.1.1:/tmp/

On the device, the sysupgrade must then be started using

    sysupgrade -n /tmp/<filename-of-squashfs-sysupgrade>.bin

Gluon image name change
=======================

The device had the image name "openmesh-mr600" in older versions of Gluon.
This had to be changed with the new name in the device trees of the ath79
device tree.

docs/user/supported_devices.rst

@@ -37,6 +37,7 @@ ath79-generic
 
   - A40
   - A60
+  - MR600 (v1)
 
 * Plasma Cloud
 

targets/ath79-generic

@@ -101,6 +101,12 @@ device('openmesh-a60', 'openmesh_a60', {
 	factory = false,
 })
 
+device('openmesh-mr600-v1', 'openmesh_mr600-v1', {
+	factory = false,
+	-- old name from OpenWrt 19.07.x
+	manifest_aliases = {'openmesh-mr600'},
+})
+
 
 -- Plasma Cloud