Introduce CONFIG option to enable/disable microbit heap allocator

- Add CONFIG option to MicroBitConfig.h
  - Add YOTTA glue for config.json mapping
  - Introduce conditional compilation into MicroBitHeapAllocator.cpp

inc/core/MicroBitConfig.h

@@ -109,6 +109,15 @@ extern uint32_t __etext;
 #define FLASH_PROGRAM_END (uint32_t) (&__etext)
 #endif
 
+//
+// If set to '1', this option enables the microbit heap allocator. This supports multiple heaps and interrupt safe operation.
+// If set to '0', the standard GCC libc heap allocator is used, which restricts available memory in BLE scenarios, and MessageBus operations
+// in ISR contexts will no longer be safe.
+//
+#ifndef MICROBIT_HEAP_ENABLED
+#define MICROBIT_HEAP_ENABLED                   1
+#endif
+
 
 // Block size used by the allocator in bytes.
 // n.b. Currently only 32 bits (4 bytes) is supported.

inc/platform/yotta_cfg_mappings.h

@@ -71,6 +71,10 @@
     #define MICROBIT_DEFAULT_PULLMODE YOTTA_CFG_MICROBIT_DAL_DEFAULT_PULLMODE
 #endif
 
+#ifdef YOTTA_CFG_MICROBIT_DAL_HEAP_ENABLED
+    #define MICROBIT_HEAP_ENABLED YOTTA_CFG_MICROBIT_DAL_HEAP_ENABLED
+#endif
+
 #ifdef YOTTA_CFG_MICROBIT_DAL_PANIC_ON_HEAP_FULL
     #define MICROBIT_PANIC_HEAP_FULL YOTTA_CFG_MICROBIT_DAL_PANIC_ON_HEAP_FULL
 #endif

source/core/MicroBitHeapAllocator.cpp

@@ -54,6 +54,8 @@ DEALINGS IN THE SOFTWARE.
 #include "MicroBitCompat.h"
 #include "ErrorNo.h"
 
+#if CONFIG_ENABLED(MICROBIT_HEAP_ENABLED)
+
 // A list of all active heap regions, and their dimensions in memory.
 HeapDefinition heap[MICROBIT_MAXIMUM_HEAPS] = { };
 uint8_t heap_count = 0;
@@ -395,3 +397,15 @@ _realloc_r (struct _reent *ptr, void *old, size_t newlen)
     (void) ptr;
     return realloc (old, newlen);
 }
+
+#else
+
+int microbit_create_heap(uint32_t start, uint32_t end)
+{
+    (void) start;
+    (void) end;
+
+    return MICROBIT_OK;
+}
+
+#endif