Compatibility changes

Make changes to improve compatibility:
-Include toolchain.h inside mbed.h
-define EXTERN

hal/api/mbed.h

@@ -18,6 +18,7 @@
 
 #define MBED_LIBRARY_VERSION 121
 
+#include "toolchain.h"
 #include "platform.h"
 
 // Useful C libraries

hal/api/toolchain.h

@@ -32,4 +32,9 @@ typedef int FILEHANDLE;
 #   define PACKED   __attribute__((packed))
 #endif
 
+// Backwards compatibility
+#ifndef EXTERN
+#define EXTERN extern
+#endif
+
 #endif