ath79: add Senao 'failsafe' sysupgrade procedure

ath79: add Senao 'failsafe' sysupgrade procedure

Use a similar upgrade method for sysupgrade.bin, like factory.bin,
for Senao boards with the tar.gz OEM upgrade platform,
and 'failsafe' image which is loaded on checksum failure.

This is inspired by the OEM upgrade script /etc/fwupgrade.sh
and the existing platforms for dual-boot Senao boards.

Previously, if the real kernel was damaged or missing
the only way to recover was with UART serial console,
because the OKLI lzma-loader is programmed to halt.

uboot did not detect cases where kernel or rootfs is damaged
and boots OKLI instead of the failsafe image,
because the checksums stored in uboot environment
did not include the real kernel and rootfs space.

Now, the stored checksums include the space for both
the lzma-loader, kernel, and rootfs.
Therefore, these boards are now practically unbrickable.

Also, the factory.bin and sysupgrade.bin are now the same,
except for image metadata.
This allows for flashing OEM image directly from openwrt
as well as flashing openwrt image directly from OEM.

Make 'loader' partition writable so that it can be updated
during a sysupgrade.

tested with
ENS202EXT v1
EAP1200H
EAP350 v1
EAP600
ECB350 v1
ECB600
ENH202 v1

Signed-off-by: Michael Pratt <mcpratt@pm.me>