poweprc/km82xx: add board specific environment variable

poweprc/km82xx: add board specific environment variable

On mgcoge3ne a new environment variable bobcatreset is used.
So this patch adds a possibility to add board specific
environment variables in general and this specific variable
for mgcoge3ne.

Signed-off-by: Holger Brunck <[email protected]>
Acked-by: Heiko Schocher <[email protected]>
cc: Wolfgang Denk <[email protected]>
cc: Detlev Zundel <[email protected]>
cc: Valentin Longchamp <[email protected]>
Signed-off-by: Valentin Longchamp <[email protected]>