arm, ppc: rework environment variables for keymile boards

This patch reworks all headerfiles for keymile boards. Furthermore
the environment variables are refactored.

Changes:
  - introduce km-powerpc.h file and extract ppc specific parts to it
  - move ARM specific options and vaiables to km_arm.h
  - sort the environment variables to logical groups
  - enhance the description of the environment variables
  - remove KM specific HW key and board id from kernel command line

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