Add mbed-os version macros

Allow compile-time tests on the version of mbed-os to cope with
e.g. API changes across versions.

Author: Colin Hogben

mbed.h

@@ -18,6 +18,14 @@
 
 #define MBED_LIBRARY_VERSION 128
 
+#define MBED_MAJOR_VERSION 5
+#define MBED_MINOR_VERSION 2
+#define MBED_PATCH_VERSION 0
+
+#define MBED_ENCODE_VERSION(major,minor,patch) ((major)*10000 + (minor)*100 + (patch))
+
+#define MBED_VERSION MBED_ENCODE_VERSION(MBED_MAJOR_VERSION,MBED_MINOR_VERSION,MBED_PATCH_VERSION)
+
 #if MBED_CONF_RTOS_PRESENT
 #include "rtos/rtos.h"
 #endif