imx: ventana: make fdt_file1 and fdt_file2 automatic and non-overridable

The fdt_file1 and fdt_file2 vars provide fallbacks for the fdt file loading
script. There is no need to allow the user to override these as if they
want to specify the fdt, they should do so in the first attempt which is
the fdt_file var.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>