Wrap using namespace in headers with codal config flag.

Flag introduced in codal-core in:
lancaster-university/codal-core@509086c

By default the `using namespace` line would still be present, as
removing it can cause some codal targets to break if they depend
on the header file to set up this namespace globally.

For example, the codal-microbit-v2 target had this update to
fix the namespacing:
lancaster-university/codal-microbit-v2#437

This patch is part of:
lancaster-university/codal-microbit-v2#240

inc/NRF52ADC.h

@@ -42,15 +42,21 @@ DEALINGS IN THE SOFTWARE.
 //
 #define NRF52_ADC_DATA_READY     1
 
-
-using namespace codal;
-
 //
 // NRF52ADCChannel status codes
 //
 #define NRF52_ADC_CHANNEL_STATUS_ENABLED                0x10
 #define NRF52_ADC_CHANNEL_STATUS_CONNECTED              0x20
 
+// This "using namespace codal" cannot be removed from this header file without
+// breaking targets where some .cpp files depend on it. So this config flag is
+// enabled by default and keeps the namespace for compatibility.
+#if CONFIG_ENABLED(CODAL_USE_GLOBAL_NAMESPACE)
+using namespace codal;
+#endif
+
+namespace codal
+{
 class NRF52ADC;
 
 class NRF52ADCChannel : public DataSource
@@ -350,5 +356,6 @@ class NRF52ADC : public CodalComponent, public PinPeripheral
      */
     void configureSampling();
 };
+} // namespace codal
 
 #endif

inc/NRF52PDM.h

@@ -40,7 +40,15 @@ DEALINGS IN THE SOFTWARE.
 //
 #define NRF52_PDM_BUFFER_SIZE           512
 
+// This "using namespace codal" cannot be removed from this header file without
+// breaking targets where some .cpp files depend on it. So this config flag is
+// enabled by default and keeps the namespace for compatibility.
+#if CONFIG_ENABLED(CODAL_USE_GLOBAL_NAMESPACE)
 using namespace codal;
+#endif
+
+namespace codal
+{
 
 class NRF52PDM : public CodalComponent, public DataSource
 {
@@ -103,5 +111,6 @@ class NRF52PDM : public CodalComponent, public DataSource
 
     void startDMA();
 };
+} // namespace codal
 
 #endif

inc/NRF52PWM.h

@@ -15,9 +15,15 @@
 #define NRF52PWM_PWM_PERIPHERALS    3
 #define NRF52PWM_PWM_CHANNELS       4
 
-
+// This "using namespace codal" cannot be removed from this header file without
+// breaking targets where some .cpp files depend on it. So this config flag is
+// enabled by default and keeps the namespace for compatibility.
+#if CONFIG_ENABLED(CODAL_USE_GLOBAL_NAMESPACE)
 using namespace codal;
+#endif
 
+namespace codal
+{
 class NRF52PWM : public CodalComponent, public DataSink, public PinPeripheral
 {
 
@@ -153,5 +159,6 @@ class NRF52PWM : public CodalComponent, public DataSink, public PinPeripheral
     int tryPull(uint8_t b);
 
 };
+} // namespace codal
 
 #endif