mx6cuboxi: Load the correct 'fdtfile' variable

Instead of hardcoding the 'fdtfile' variable, let's detect the SoC and
board variant on the fly and change the dtb name.

Based on the scheme done on am335x board.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Tested-By: Vagrant Cascadian <vagrant@debian.org>
Reviewed-by: Tom Rini <trini@konsulko.com>