km/common: rework and simplify default environment

This is the second step to simplify and decrease the default
environment for the keymile boards. The release usecase formaly
used to set the production environment was removed and the default
configuration is now the production environment. So the formar
environment variable "release" which has done a lot of things
in the past, simply erase the current environment and do a reset
which forces u-boot to setup the default environment again.

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