km/common: simplify default environment

This is a first step to simplify the default environment. Move all
the environment variables which are only needed for debugging
purpose to textfiles in the scripts directory. In case of debugging
these files can be loaded via tftp into RAM and set via the env import
command. Other variables are identified as obsolete and were removed.

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