ARM: OMAP4/5: Change the default boot command to work with device tree

Now with kernel moving to all device tree, the default
boot command is changed to pass the device tree blob.
Also, adding the findfdt command to get the dt-blob
based on the board.

Thanks to Tom Rini <trini@ti.com> for suggesting this.

Signed-off-by: Sricharan R <r.sricharan@ti.com>