Add support for sdcard images

Create also sdcard images that can be easily dd'ed to memory card.
Image type created for imx6 works also in beagleboard when
first partition is marked as bootable.

Change-Id: I45323de442187c67867be31f80f1cdef43a12a9e
Reviewed-by: Kalle Viironen <kalle.viironen@digia.com>