powerpc: common updates for keymile boards

- added to keymile-common.h:
  - bootcount support
  - COMMAND HISTORY
  - CONFIG_AUTO_COMPLETE
  - CONFIG_SYS_FLASH_PROTECTION
  - JFFS2 support
  - CONFIG_VERSION_VARIABLE
- extracted common I2C settings for all boards
- common default environment settings summarized

Signed-off-by: Heiko Schocher <hs@denx.de>