[platform][zynq] fix stall condition in GEM transmit

In a particular situation where a tx packet was queued and a rx packet was received just
afterwards, but before the tx was sent, the tx interrupt would be acked and
the tx event would never be signalled.
[app][inetsrv] increase discard buffer size for efficiency
[lib][minip] mark dhcp's thread as detached so it cleans up after itself