pxe: prepend fdtdir to DTB name irrespective of source

The directory name from an fdtdir directive in a PXE config file should
always be pre-pended to the DTB filename; it shouldn't matter whether
the DTB filename came from the $fdtfile environment variable, or whether
it was constructed dynamically from ${soc}-${board}.dtb. Fix the code to
always prepend the directory name.

Reported-by: Dennis Gilmore <dennis@ausil.us>
Fixes: c61d94d8 ("pxe: implement fdtdir extlinux.conf tag")
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Dennis Gilmore <dennis@ausil.us>
Tested-by: Dennis Gilmore <dennis@ausil.us>