Add support for loading and saving the environment to a FAT partition

The following must be defined:

CONFIG_ENV_IS_IN_FAT
	Enable this saving environment to FAT.

FAT_ENV_INTERFACE
	Interface the FAT resides on (e.g. mmc).

FAT_ENV_DEVICE
	The interface device number (e.g. 0 for mmc0)

FAT_ENV_PART
	The device part (e.g. 1 for mmc0:1)

FAT_ENV_FILE
	The filename of the environment file.

Author:    Maximilian Schwerin <mvs@tigris.de>

Removed dead DEBUG comment.
Signed-off-by: Wolfgang Denk <wd@denx.de>