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 namespace to be set up 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
 
-
+// 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
 
 //
 // NRF52ADCChannel status codes
 //
 #define NRF52_ADC_CHANNEL_STATUS_ENABLED                0x10
 #define NRF52_ADC_CHANNEL_STATUS_CONNECTED              0x20
 
+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