OMAP3: igep00x0: allow booting with a FDT from MMC

IGEP boards now have Device Tree support in the mainline
kernel. To boot an IGEP board using a DT, a uEnv.txt plain
text file could be used to define a custom uenvcmd that will
be run by the default boot command.

It is more convenient to change the default boot command to
allow loading a FDT if it is stored in the boot dir of the
rootfs uSD/MMC partition.

If no FDT is found then the defaul command tries to boot a
zImage without a DT using legacy boot.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>