[U-Boot] Version 2 of Kirkwood and env_nand improvements

Respin of my patch series for review. Only changes in patches 2 and 3:

env_nand.c: support falling back to redundant env when writing: Print status message for each of the redundant copies when writing. So when writing, we see which image is being written to. Also, when the first attempt fails, a second message follows so the user sees that writing was eventually successful.

env_nand.c: clarify log messages when env reading fails: Print a warning if one of the two redundant copies could not be read and an error if both failed to read. Message text and formatting chosen with common/env_flash.c as example.