Add second level of preempt priorities

With previous changes in the Zephyr configuration
and Web services controllers, this allows file uploads
and downloads to function while fixing the simultaneous
connection stall problem seen previously.

prj.conf

@@ -34,9 +34,9 @@ CONFIG_NET_TC_THREAD_PREEMPTIVE=y
 CONFIG_NET_TC_TX_COUNT=1
 
 # The Kestrel CPU is quite slow, and it's easy to starve the CivetWeb process
-# during file uploads if the TCP queues have a higher preempt priority than
-# the CivetWeb process (by default, this is true)...
-CONFIG_NUM_PREEMPT_PRIORITIES=1
+# during file uploads if the TCP queues have a significantly higher
+# priority than the CivetWeb process (by default, this is true)...
+CONFIG_NUM_PREEMPT_PRIORITIES=2
 
 # Kernel options
 CONFIG_TEST_RANDOM_GENERATOR=y