kwbimage: Add support for SDIO/MMC boot device selection

This patch adds support to select the "sdio" as boot device in the
kwbimage.cfg file. This line selects this SDIO device:

BOOT_FROM  sdio

Tested on Marvell DB-88F6820-GP board.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Cc: Dirk Eibach <eibach@gdsys.de>