PXELINUX: Use sendto() instead of connect()/send()/disconnect()

PXELINUX: Use sendto() instead of connect()/send()/disconnect()

This commit prevents a race-condition on systems that have functional
interrupts (observed with iPXE and select other Dell systems).  Without
this, the reply packet could be received by the core prior to the
disconnect() call, see that it doesn't have a matching PCB (protocol
control block, iirc) since the reply has a different far-end UDP port
than the original request, and lwIP will discard the packet before
PXELINUX can see it.

net_core_sendto() instead of
net_core_connect() net_core_send() net_core_disconnect()

Commit message expanded with Matt Fleming's assistance

Signed-off-by: Gene Cumm <gene.cumm@gmail.com>