arm: omap: Add check for fdtfile in the findfdt macro

In the omap4, omap5 and am335x common files add a check to ensure that the fdtfile is
defined after the findfdt macro has run.  If the file is not defined then warn the user that the
dtb file is not defined.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Reviewed-by: Tom Rini <trini@ti.com>