microbit: Setting of default BLE transmission power level

Added configuration option in MicroBitConfig to allow the default power level
to be defined. Based on experiments undertaken by the BBC and subsequent policies defined by
the BBC's child protection panel, to is set by default it's lowest level (0),
in order to protect children's privacy.

It should be noted that this is 30dB below the device's default setting, so will
inevitably leave the device more susceptible to interference, and therefore
significantly reduce the reliability of the device. This will likely present
itself through increased packet loss, reduced thorughout, lost data and
connection drop outs. The normal BLE power level on commercial devices
translates to  power level of 6.

The author has witnessed an inability to connect to a micro:bit from a
distance of 40cm under domestic conditions using a Samsung S5 mini
running Android 4.4.2.

inc/MicroBitConfig.h

@@ -157,6 +157,14 @@
 #define MICROBIT_BLE_ADVERTISING_TIMEOUT    0
 #endif
 
+// Defines default power level of the BLE radio transmitter.
+// Valid values are in the range 0..7 inclusive, with 0 being the lowest power and 7 the highest power.
+// Based on trials undertaken by the BBC, the radio is normally set to its lowest power level
+// to best protect children's privacy. 
+#ifndef MICROBIT_BLE_DEFAULT_TX_POWER
+#define MICROBIT_BLE_DEFAULT_TX_POWER       0
+#endif
+
 // Enable/Disable BLE Service: MicroBitDFU
 // This allows over the air programming during normal operation.
 // Set '1' to enable. 

source/ble-services/MicroBitBLEManager.cpp

@@ -154,6 +154,9 @@ void MicroBitBLEManager::init(ManagedString deviceName, ManagedString serialNumb
     ble->gap().setAdvertisingPolicyMode(Gap::ADV_POLICY_FILTER_CONN_REQS);
 #endif    
 
+    // Configure the radio at our default power level
+    setTransmitPower(MICROBIT_BLE_DEFAULT_TX_POWER); 
+
     // Bring up any configured auxiliary services.
 #if CONFIG_ENABLED(MICROBIT_BLE_DFU_SERVICE)
     new MicroBitDFUService(*ble);