Changes for FreeRTOS library

Ensure all the priority bits are assigned to be preempt priority
bits by calling NVIC_PriorityGroupConfig( NVIC_PriorityGroup_4 );
before the RTOS is started.

See https://www.freertos.org/RTOS-Cortex-M3-M4.html

Signed-off-by: Frederic.Pillon <frederic.pillon@st.com>

Author: fpistm

cores/arduino/main.cpp

@@ -29,7 +29,13 @@ void initVariant() { }
 // Force init to be called *first*, i.e. before static object allocation.
 // Otherwise, statically allocated objects that need HAL may fail.
  __attribute__(( constructor (101))) void premain() {
-    init();
+
+// Required by FreeRTOS, see http://www.freertos.org/RTOS-Cortex-M3-M4.html
+#ifdef NVIC_PRIORITYGROUP_4
+  HAL_NVIC_SetPriorityGrouping(NVIC_PRIORITYGROUP_4);
+#endif
+
+  init();
 }
 
 /*