omap3_logic: Rework MACH_TYPE and fdtfile logic

The MACH_TYPE values for the omap37xx based platforms are no longer
officially valid, so we must not set and pass them.  In order to not
reference them but still be able to set the default fdtfile based on the
board detection logic we need to combine the two steps into one.

Cc: Adam Ford <aford173@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Adam Ford <aford173@gmail.com>