While barebox itself should be on the eMMC boot partitions eventually
it's nice to have this set up, so one can dd a prebuilt SD card image
to the eMMC and have something working. A working Barebox env is also
very convinient to have around during development.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>