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 <stefan.agner@toradex.com>
Acked-by: Max Krummenacher <max.krummenacher@toradex.com>
Acked-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Igor Opaniuk <igor.opaniuk@toradex.com>