mx6: mx6sabre common: Use more conventional spelling of fdtfile

Both the distro boot configuration as well as the general pxe support
standardized on fdtfile as the variable to hold the dtb filename.
Adjust to follow that such that e.g. "pxe boot" will just work.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>