Making fetch to work with file on sub dir on tftp server.

Making fetch to work with file on sub dir on tftp server.

This patch allows live-initramfs to fetch file from sub
dir on tftp server. The original one from live-initramfs 1.157.2 only
works for the file existing on the root of tftp server, e.g.
fetch=tftp://192.168.101.254/filesystem.squashfs

It allows the PXE client to get the file either on the root, or on
the sub dir, e.g. fetch=tftp://192.168.101.254/live/filesystem.squashfs
is working now.