microbit: Increase maximum descriptor count

The ble-nrf51822 library maintains a static array of BLE descriptors (by
default, 8). This patch increases the size of this array to 20. This is
required to hold the full complement of services that make up the micro:bit
standard BLE profile.

source/nRF5xGattServer.h

@@ -45,7 +45,7 @@ public:
 
 private:
     const static unsigned BLE_TOTAL_CHARACTERISTICS = 30;
-    const static unsigned BLE_TOTAL_DESCRIPTORS     = 8;
+    const static unsigned BLE_TOTAL_DESCRIPTORS     = 20;
 
 private:
     /**