add monitor mode debugging support

src/nrf52/main-nrf52.cpp

@@ -75,6 +75,10 @@ void nrf52Setup()
     // per https://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.nrf52832.ps.v1.1%2Fpower.html
     DEBUG_MSG("Reset reason: 0x%x\n", why);
 
+    // Per https://devzone.nordicsemi.com/nordic/nordic-blog/b/blog/posts/monitor-mode-debugging-with-j-link-and-gdbeclipse
+    // This is the recommended setting for Monitor Mode Debugging
+    NVIC_SetPriority(DebugMonitor_IRQn, 6UL);
+
     // Not yet on board
     // pmu.init();
     DEBUG_MSG("FIXME, need to call randomSeed on nrf52!\n");