[autoboot] Enable infrastructure to specify an autoboot device location

[autoboot] Enable infrastructure to specify an autoboot device location

iPXE will currently attempt to boot from every network device for
which it has a driver.  Where a system has more than one network
device supported by iPXE, this renders BIOS IPL lists ineffective.

Allow an autoboot device location to be specified.  If such a location
is specified, then only devices matching that location will be used as
part of the automatic boot sequence.  If no such location is
specified, then all devices will be used.

Note that this does not affect the "autoboot" command, which will
continue to use all devices.

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