colibri_vf: add distroboot support

colibri_vf: add distroboot support

Add support for distro boot. This is especially helpful for external
devices. There is a global boot command which scans a predefined
list of boot targets:
  run distro_bootcmd

As well as direct boot commands such as:
  run bootcmd_mmc0
  run bootcmd_usb
  run bootcmd_dhcp
  ...

Refer to doc/README.distro fo details.

While at it also re-order boot command macros as well as the
CONFIG_EXTRA_ENV_SETTINGS.

Signed-off-by: Stefan Agner <[email protected]>
Acked-by: Max Krummenacher <[email protected]>
Acked-by: Marcel Ziswiler <[email protected]>
Reviewed-by: Igor Opaniuk <[email protected]>