pxeboot: make the tftp loader use the option root-path directive

pxeboot in tftp loader mode (when built with LOADER_TFTP_SUPPORT) now
prefix all the path to open with the path obtained via the option 'root-path'
directive.

This allows to be able to use the traditional content /boot out of box. Meaning
it now works pretty much like all other loaders. It simplifies hosting hosting
multiple version of FreeBSD on a tftp server.

As a consequence, pxeboot does not look anymore for a pxeboot.4th (which was
never provided)

Note: that pxeboot in tftp loader mode is not built by default.

Reviewed by: rpokala
Relnotes: yes
Sponsored by: Gandi.net
Differential Revision: https://reviews.freebsd.org/D4590