[dhcp] Honor PXEBS_SKIP option in discovery control

[dhcp] Honor PXEBS_SKIP option in discovery control

It is permissible for a DHCP packet containing PXE options to specify
only "discovery control", instead of the more typical boot menu +
prompt options. This is the strategy used by older versions of
dnsmasq; by specifying the discovery control as PXEBS_SKIP, they cause
vendor PXE ROMs to ignore boot server discovery and just use the
filename and next-server options in the initial (Proxy)DHCP packet.
Modify iPXE to accept this behavior, to be more compatible with the
Intel firmware.

Signed-off-by: Joshua Oreman <oremanj@rwcr.net>
Tested-by: Kyle Kienapfel <kyle@shadowmage.org>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>