net: ip: Allow traffic class thread priority customization

Add traffic class priority custom option to customize traffic class
priority base for throughput performace.

Wi-Fi traffic performance depends on cooperation between net threads
and Wi-Fi driver threads. So we want traffic class threads priority
to be more flexible to fit more vendors.

Eg: when traffic class tx thread priority is higher than driver tx
thread, it will consume much more cpu time and send packets until driver
queue full.
When traffic class tx thread priority is lower than driver tx thread,
it will become serial transmition, also affecting throughput.

Signed-off-by: Fengming Ye <frank.ye@nxp.com>