karo: tx53: add 'romupdate' command to update the U-Boot image in flash

Implement the romupdate command, that is already in use on TX28 and
TX6.
The command searches for a sufficiently large area without bad blocks
inside the u-boot (or any given other) partition and dynmaically
creates the FCB that is required to boot from flash.

It is also possible to write a redundant image, that will be used if
the first block of the normal image cannot be read due to ECC errors.