uboot-imx

udoo_neo: Use 'fdtfile' variable name

'fdtfile' is the preferred name for the variable that contains the
device tree blob according to the README file.

It also makes it consistent with other i.MX boards that use config_distro,
so change it accordingly.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>