gk802: cleanup bootcmd and add recovery switch

Boot order:
first on MMC1 then MMC0, loading from EXT[234] filesystems:
    - try and source script /boot/ubootcmd
    - try and load kernel /boot/uImage

If this all fails, try again looking for ubootcmd_recovery and
uImage_recovery.

If recovery switch is held, bootdelay is set to 5, and the board
tries the recovery files first.