Add FIT image booting from MMC device, during MMC bootcmd
u-boot env script look for bootscript, else fit image or else
finally look for legacy image uImage.

Cc: Stefano Babic <sbabic@denx.de>
Cc: Matteo Lisi <matteo.lisi@engicam.com>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>