[PATCH] Fix ppc4xx bootstrap letter displayed on startup

The attached patch is mainly cosmetic, allowing u-boot to
display the correct bootstrap option letter according to the
datasheets.

The original patch was extended with 405EZ support by Stefan
Roese.

Signed-off-by: Benoit Monin <bmonin@adeneo.eu>
Signed-off-by: Stefan Roese <sr@denx.de>