ramips: fix partitions and boot for RAVPower RP-WD03

The RAVPower RP-WD03 is a battery powered router, with an Ethernet and
USB port. Due due a limitation in the vendor supplied U-Boot bootloader,
we cannot exceed a 1.5 MB kernel size, as is the case with recent builds
(i.e. post v19.07). This breaks both factory and sysupgrade images.

To address this, use the lzma loader (loader-okli) to work around this
limitation.

The improvements here also address the "misplaced" U-Boot environment
partition, which is located between the kernel and rootfs in the stock
image / implementation. This is addressed by making use of mtd-concat,
maximizing space available in the booted image.
This will make sysupgrade from earlier versions impossible.

Changes are based on the recently supported HooToo HT-TM05, as the
hardware is almost identical (except for RAM size) and is from the same
vendor (SunValley). While at it, also change the SPI frequency
accordingly.

Installation:

 - Download the needed OpenWrt install files, place them in the root
   of a clean TFTP server running on your computer. Rename the files as,
   - openwrt-ramips-mt7620-ravpower_rp-wd03-squashfs-kernel.bin => kernel
   - openwrt-ramips-mt7620-ravpower_rp-wd03-squashfs-rootfs.bin => rootfs
 - Plug the router into your computer via Ethernet
 - Set your computer to use 10.10.10.254 as its IP address
 - With your router shut down, hold down the power button until the first
   white LED lights up.
 - Push and hold the reset button and release the power button. Continue
   holding the reset button for 30 seconds or until it begins searching
   for files on your TFTP server, whichever comes first.
 - The router (10.10.10.128) will look for your computer at 10.10.10.254
   and install the two files. Once it has finished installation, it will
   automatically reboot and start up OpenWrt.
 - Set your computer to use DHCP for its IP address

Notes:

 - U-Boot environment can be modified, u-boot-env is preserved on initial
   install or sysupgrade
 - mtd-concat functionality is included, to leave a "hole" for u-boot-env,
   combining the OEM kernel and rootfs partitions

Most of the changes in this commit are the work of Russell Morris (as
credited below), I only wrapped them up and added compat-version.
Thanks to @mpratt14 and @xabolcs for their help getting the lzma loader
to work!

Fixes: 5ef79af4f80f ("ramips: add support for Ravpower WD03")

Suggested-by: Russell Morris <rmorris@rkmorris.us>
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

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