+fdtdir <path> - if this label is chosen, use tftp to retrieve a fdt blob
+ relative to <path>. If the fdtfile environment variable
+ is set, <path>/<fdtfile> is retrieved. Otherwise, the
+ filename is generated from the soc and board environment
+ variables, i.e. <path>/<soc>-<board>.dtb is retrieved.
+ If the fdt command is specified, fdtdir is ignored.
+