Field Fail-Save U-boot Update
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Field Fail-Save u-boot update is a feature that allows save u-boot update
of FOX and XMC products that are rolled out in the field.

The feature is initially implemented for designs based on LS102x SoC, but in
theory can be used on all designs that are booting from parallel NOR flash.

The implementation expects redundant (secondary) u-boot image on a predefined
location in the NOR flash, u-boot execution will be transferred to the redundant 
(secondary) u-boot and redundant u-boot will be started if 'updateduboot' envvar
is set to 'yes'.
Update logic check_for_uboot_update() has to be invoked from the design early
before relocation just after SoC initialization, e.g from board_early_init_f or
misc_init_f functions.
By design it is expected that primary u-boot image is burned in the factory and
never updated, and in case u-boot update is required it can flashed and started
from secondary u-boot location.
