omap3: overo: Select fdtfile for expansion board

The u-boot Overo board actually supports both Overo (OMAP35xx)
and Overo Storm (AM/DM37xx) COMs with a range of different expansion
boards.  This provides a mechanism to select the an appropriate device
tree file based on the processor version and, if available, the
expansion board ID written on the expansion board EEPROM. To match the
3.15+ kernels, fdtfile names have this format:
 "omap3-overo[-storm]-<expansion board name>.dtb"

By default, we use "omap3-overo-storm-tobi.dtb".

Signed-off-by: Ash Charles <ashcharles@gmail.com>
Conflicts:
	include/configs/omap3_overo.h