omap3_beagle: support booting from zImage and device tree as last option

If no other bootoption works, try loading up device tree and zImage.

This is selected as the last option to allow backward compatibility as
well as support the recent trend in moving kernel boot to using zImage
and device tree.

NOTE: if uImage is present in bootpart, it will try this first and
will assume this is to be booted with bootm (so may be concatenated
image or plain vanilla ATAG MACHINE_ID based image)

Signed-off-by: Nishanth Menon <nm@ti.com>