[autoboot] Use next-server from filename's settings block

Locate the settings block containing the filename, and search only
that settings block for the next-server address.  This avoids problems
caused by misconfigured DHCP servers which provide a next-server
address (often defaulting to the DHCP server's own IP address) even
when not providing a filename.

Originally-implemented-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>