net: Enable running without TX or RX threads

Set the default behaviour of the networking subsystem so that
no TX or RX threads are created. This will save RAM as there
is no need to allocate stack space for the RX/TX threads.
Also this will give small improvement to network packet latency
shown here:
* with 1 traffic class (1 TX and RX thread)

Avg TX net_pkt (42707) time 60 us	[0->22->15->22=59 us]
Avg RX net_pkt (42697) time 36 us	[0->10->3->12->7=32 us]

* with 0 traffic classes (no TX and RX threads)

Avg TX net_pkt (41608) time 42 us	[0->21->20=41 us]
Avg RX net_pkt (41593) time 31 us	[0->9->12->8=29 us]

In this qemu_x86 test run, 40k UDP packets was transferred between
echo-server and echo-client. In TX the speed increase was 30% and
in RX it was 14%.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>