omap3_beagle: support findfdt and loadfdt for devicetree support

For folks not using concatenated device tree with uImage, having
an handy function to find and load device tree is very handy.

So introduce findfdt and loadfdt and run findfdt by default to make
it easier on user scripts.

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