[nrf noup] Increase Main thread priority to fix an app core crash

Currently, we have set 0 priority both to the main thread and
the nrf53 cpuapp IPC0 priority. When we perform a long operation
on the main thread we block IPC communication and get a crash on
app core when it tries to send a packet through ieee820154 radio.

To avoid that situation and the crash we can increase the main
thread priority to 1.

Author: ArekBalysNordic

config/nrfconnect/chip-module/Kconfig.defaults

@@ -449,6 +449,13 @@ config BOOT_IMAGE_ACCESS_HOOKS
 config UPDATEABLE_IMAGE_NUMBER
     default 2
 
+# Change the main thread priority to 1 to avoid setting the same thread 
+# priority value as for IPC0 (which default value is set to 0). 
+# If the priorities are equal long operations on the main thread can
+# block IPC communication to the network core and cause application crash.
+config MAIN_THREAD_PRIORITY
+    default 1
+
 # Generate random numbers using Xoroshiro algorithm instead of direct calls
 # to the cryptocell library to workaround firmware hangs.
 choice RNG_GENERATOR_CHOICE