[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>
[prefix] Allow prefix to specify a PCI autoboot device location

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
[romprefix] Store boot bus:dev.fn address as autoboot device location

Per the BIOS Boot Specification, the initialization phase of the ROM
is called with the PFA (PCI Function Address) in the %ax register.
The intention is that the ROM code will store that device address
somewhere and use it for booting from that device when the Boot Entry
Vector (BEV) is called.  iPXE does store the PFA, but doesn't use it
to select the boot network device.  This renders BIOS IPL lists fairly
ineffective.

Fix by using the BBS-specified bus:dev.fn address as the autoboot
device location.

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