uboot-mvebu: add support for MikroTik RB5009

uboot-mvebu: add support for MikroTik RB5009

MikroTik RB5009 uses RouterBoot as its bootloader like all MikroTik devices
running RouterOS, meaning that its not FIT compatible and can only boot
ELF images.

Now this is not so much of an issue on ARM or MIPS since kernel supports
appending DTB-s to it (Or we patch the kernel to embed it), but on ARM64
there is intentionally no such support.

RouterBoot will pass a DTB, but its the broken MikroTik one which is a
modified reference DTB and incorrect in more places than its valid so we
cannot use it to boot our kernel.

Thus, the solution is to use an intermediary loader and luckily for us
Armada 7040 is well supported in U-Boot which makes it a great option since
it supports anything that we will ever need to boot.

Upstream U-Boot currently requires the Armada boards to be converted to
OF_UPSTREAM before adding anything new and this requires updating all of
the drivers to accomodate the Linux DTS, while I plan to do this eventually
we will need to keep this board downstream for now.

Most stuff is supported in U-Boot, including networking since the switch
is preconfigured by RouterBoot.

A custom environment is used to try and boot from the following devices:
1. NAND (UBI)
2. USB
3. Networking

If NAND boot fails then U-Boot will attempt to boot OpenWrt initramfs from
USB or via networking.

There is a manual recovery mechanism implemented where if the reset button
is held when U-Boot is booting it will try to boot OpenWrt initramfs from:
1. USB
2. Networking

When U-Boot is in recovery mode it will light all of the LED-s except the
switch ones.

Link: https://github.com/openwrt/openwrt/pull/15765
Signed-off-by: Robert Marko <robimarko@gmail.com>