ath79: Fix for Mikrotik NAND bad blocks - testers needed

[Danijel]

> I have a few RB493G boards that will not boot OpenWrt due to the bad
> blocks problem. Might your work work with them?
Yes, if you first return to RouterOS (to create a proper kernel partition),
you can build a new image with my changes, boot initramfs and try
flashing the sysupgrade to NAND. The modified sysupgrade process
mounts the YAFFS2 kernel partition and replaces the kernel file,
instead of blindly overwriting the partition on the block level.

> I would be willing to split shipping
> and charge no more than that (I am in the USA).
I'm in the EU so maybe we should start with you testing a few
problematic boards yourself before shipping them, if you can.