env_nand.c: Added bad block management for environment variables

Modified to check for bad blocks and to skipping over them when
CFG_ENV_RANGE has been defined.
CFG_ENV_RANGE must be larger than CFG_ENV_SIZE and aligned to the NAND
flash block size.

Signed-off-by: Stuart Wood <stuart.wood@labxtechnologies.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>