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

[Phil Sutter]

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.