pumpkin: Set the mmcboot environment variable to boot android

This adds a new definition of MMCBOOT, which could be selected
at build time, to boot Android, using AVB.

 Conflicts:
	include/configs/pumpkin.h

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
2 files changed