PXE is broken on LWIP

[Heinrich Schuchardt]

Hello Jerome

PXE booting from standard boot with NET_LWIP just stalls if the server 
does not respond:

Using ethernet at 16030000 device
TFTP from server 192.168.99.1; our IP address is 192.168.99.47
Filename 'pxelinux.cfg/01-6c-cf-39-00-2d-29'.
Load address: 0x45900000
Loading:

If a server cannot be connected, the network loop should be exited.

When pressing CTRL-C multiple times I see crashes with a call chain

tftp_open -> sys_check_timeouts -> memp_free -> do_memp_free_pool

I guess in sys_check_timeouts() we must remove timed-out timers from the 
chain to avoid freeing them twice.

Best regards

Heinrich