uknetdev: Yield receive thread on full stack queue

uknetdev: Yield receive thread on full stack queue

When a too high rate of network traffic is received, the netdev's receive
thread may never yield and only forwards packets to the lwip stack
thread. This works as expected until the input queue runs full. If the
the stack is never able to process them, the stack becomes completely
unresponsive. Instead of dropping just a few packets all successive
packets are dropped.

This commit introduces a thread yield when this condition is met and
gives the chance to process queued packets.

Signed-off-by: Simon Kuenzer <simon.kuenzer@neclab.eu>
Reviewed-by: Felipe Huici <felipe.huici@neclab.eu>