WinPE's pxeboot.n12 takes the BufferLimit returned by gPXE (indicating
the size of gPXE's internal DHCP packet buffers) and erroneously passes
it in as BufferSize (indicating the size of pxeboot.n12's DHCP packet
buffer).  If these don't match, then pxeboot.n12 ends up instructing gPXE
to overwrite parts of its data segment.

Change gPXE's internal DHCP packet buffers to be exactly
sizeof(BOOTPLAYER_t) bytes to work around this problem.