ath79: add support for Meraki MR12

ath79: add support for Meraki MR12

Port device support for Meraki MR12 from the ar71xx target to ath79.

Specifications:

  - SoC: AR7242-AH1A CPU
  - RAM: 64MiB (NANYA NT5DS32M16DS-5T)
  - NOR Flash: 16MiB (MXIC MX25L12845EMI-10G)
  - Ethernet: 1 x PoE Gigabit Ethernet Port (SoC MAC + AR8021-BL1E PHY)
  - Ethernet: 1 x 100Mbit port (SoC MAC+PHY)
  - Wi-Fi: Atheros AR9283-AL1A (2T2R, 11n)

Installation:

  1. Requires TFTP server at 192.168.1.101, w/ initramfs & sysupgrade .bins
  2. Open shell case
  3. Connect a USB->TTL cable to headers furthest from the RF shield
  4. Power on the router; connect to U-boot over 115200-baud connection
  5. Interrupt U-boot process to boot Openwrt by running:
       setenv bootcmd bootm 0xbf0a0000; saveenv;
       tftpboot 0c00000 <filename-of-initramfs-kernel>.bin;
       bootm 0c00000;
  6. Copy sysupgrade image to /tmp on MR12
  7. sysupgrade /tmp/<filename-of-sysupgrade>.bin

Notes:

  - kmod-owl-loader is still required to load the ART partition into the
    driver.

  - The manner of storing MAC addresses is updated from ar71xx; it is
    at 0x66 of the 'config' partition, where it was discovered that the
    OEM firmware stores it. This is set as read-only. If you are
    migrating from ar71xx and used the method mentioned above to
    upgrade, use kmod-mtd-rw or UCI to add the MAC back in. One more
    method for doing this is described below.

  - Migrating directly from ar71xx has not been thoroughly tested, but
    one method has been used a couple of times with good success,
    migrating 18.06.2 to a full image produced as of this commit. Please
    note that these instructions are only for experienced users, and/or
    those still able to open their device up to flash it via the serial
    headers should anything go wrong.

    1) Install kmod-mtd-rw and uboot-envtools
    2) Run `insmod mtd-rw.ko i_want_a_brick=1`
    3) Modify /etc/fw_env.config to point to the u-boot-env partition.
       The file /etc/fw_env.config should contain:

       # MTD device   env offset  env size    sector size
       /dev/mtd1      0x00000     0x10000     0x10000

       See https://openwrt.org/docs/techref/bootloader/uboot.config
       for more details.

    4) Run `fw_printenv` to verify everything is correct, as per the
       link above.
    5) Run `fw_setenv bootcmd bootm 0xbf0a0000` to set a new boot address.
    6) Manually modify /lib/upgrade/common.sh's get_image function:
       Change ...

       cat "$from" 2>/dev/null | $cmd

       ... into ...

       (
         dd if=/dev/zero bs=1 count=$((0x66)) ; # Pad the first 102 bytes
         echo -ne '\x00\x18\x0a\x12\x34\x56'  ; # Add in MAC address
         dd if=/dev/zero bs=1 count=$((0x20000-0x66-0x6)) ; # Pad the rest
         cat "$from" 2>/dev/null
       ) | $cmd

       ... which, during the upgrade process, will pad the image by
       128K of zeroes-plus-MAC-address, in order for the ar71xx's
       firmware partition -- which starts at 0xbf080000 -- to be
       instead aligned with the ath79 firmware partition, which
       starts 128K later at 0xbf0a0000.

    7) Copy the sysupgrade image into /tmp, as above
    8) Run `sysupgrade -F /tmp/<sysupgrade>.bin`, then wait

    Again, this may BRICK YOUR DEVICE, so make *sure* to have your
    serial cable handy.

Signed-off-by: Martin Kennedy <hurricos@gmail.com>
[add LED migration and extend compat message]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>