boot fallback slot type for nor flash

[esden]

Is your feature request related to a problem? Please describe.
As far as I can tell the boot-raw-fallback slot type does not work for /dev/mtd* partitions.

Describe the solution you'd like
Implement boot-nor-fallback handler. The issue with that is that flashcp does not support write offsets as far as I can tell. The alternative would be to provide two mtd partitions set up in device tree as the primary and secondary targets. But for that the RaucSlot structure might need to be expanded to pass through those parameters.

Describe alternatives you've considered
I have not tried alternatives, as I need to be able to have an atomic update of a zynq boot.bin that has to be on the QSPI flash.

If there is a different way to implement atomic QSPI bootloader updates on zynq that would be great. Then this feature is not necessary.

[jluebbe]

The PR implementing this feature has some review feedback that would need to be addressed.