Eliminate noreturn compiler warnings (#467)

Adds noreturn attribute to microbit_panic(), to prevent gcc warning
about missing returns in various functions that use panic.

Author: kevinawalsh

inc/MicroBitDevice.h

@@ -110,7 +110,7 @@ namespace codal
      * microbit_panic(20);
      * @endcode
      */
-    void microbit_panic(int statusCode);
+    [[noreturn]] void microbit_panic(int statusCode);
 
     /**
      * Defines the length of time that the device will remain in a error state before resetting.

target-locked.json

@@ -75,4 +75,4 @@
     "show_size": true,
     "snapshot_version": "v0.2.69",
     "toolchain": "ARM_GCC"
-}
+}