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 <holger.brunck@keymile.com>
Acked-by: Heiko Schocher <hs@denx.de>
cc: Wolfgang Denk <wd@denx.de>
cc: Detlev Zundel <dzu@denx.de>
cc: Valentin Longchamp <valentin.longchamp@keymile.com>
Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>