[pxe] Add a dummy PXENV+ entry point at UNDI_CS:0000
Wyse Streaming Manager server (WLDRM13.BIN) assumes that the PXENV+
entry point is at UNDI_CS:0000; apparently, somebody at Wyse has
difficulty distinguishing between the words "may" and "must"...
Add a dummy entry point at UNDI_CS:0000, which just jumps to the
correct entry point.
IBM's iSCSI Firmware Initiator checks the UNDIROMID pointer in the
!PXE structure that gets created by the UNDI loader. We didn't
previously fill this value in.