buildroot

board/minnowboard-max: Rework to generate SD card image

Let's rework the board and config files to use genimage
to generate the SD card image directly.

[Peter: add host-mtools for genimage vfat handling]
Signed-off-by: Ezequiel García <ezequiel@vanguardiasur.com.ar>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>