Use redundant environment

[spiccinini]

To support power loss when updating the environment a redundant environment should be used.
u-boot has the config CFG_REDUNDAND_ENVIRONMENT for this:

	- CFG_ENV_ADDR_REDUND
	  CFG_ENV_SIZE_REDUND

	   These settings describe a second storage area used to hold
	   a redundand copy of the environment data, so that there is
	   a valid backup copy in case there is a power failure during
	   a "saveenv" operation.

It must be tested and an upgrade path from the current layout provided (probably using the previous sector to the ART)