apalis_imx6: add distroboot support

apalis_imx6: 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_mmc1
  run bootcmd_mmc2
  run bootcmd_usb
  run bootcmd_dhcp
  ...

Refer to doc/README.distro for details.

While at it, remove the CONFIG_DRIVE_TYPES define which has not
been used and was meant to be used for multi device boot support
which is now provided by distroboot.

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>