Pull common files from pxt-common-packages (#1165)

libs/core/platform.h

@@ -0,0 +1,13 @@
+// helpful define to handle C++ differences in package
+#define PXT_MICROBIT_TAGGED_INT 1
+
+// cross version compatible way of access data field
+#ifndef PXT_BUFFER_DATA
+#define PXT_BUFFER_DATA(buffer) buffer->data
+#endif
+
+#ifndef PXT_CREATE_BUFFER
+#define PXT_CREATE_BUFFER(data, len) pxt::mkBuffer(data, len)
+#endif
+
+#define PXT_POWI 1