[RFC] Added simple hack to ignore the environment variables in flash.

I'm looking for a way to get u-boot to ignore the environment variables in flash
if there is an autoscript on the SD card. The easiest way I can find is to simply
ignore the environment variables completely and boot u-boot itself off of the SD

I see there are some other CONFIG's that do somewhat the same thing, but not

On a Beagle Board, the experience is:
* No SD card with u-boot.bin, boot u-boot.bin w/o OVERRIDE and read environment
  from flash.
* SD card with u-boot.bin w/ OVERRIDE is read. Environment in flash is ignored
  so default environment is used. The board defaults cause it to read an
  autoscr file from the SD card, allowing you to always boot, even if the
  environment variables in the flash are bad and there is no access to the
  serial console.

Any comments on if the below can be a suitable approach?

Signed-off-by: Jason Kridner <jkridner@beagleboard.org>