Include events library headers in mbed.h

The inclusion is conditioned by the presence of the events library
(`MBED_CONF_EVENTS_PRESENT`). This ensures backward compatibility with
SDK builds.

events/mbed_lib.json

@@ -0,0 +1,6 @@
+{
+    "name": "events",
+    "config": {
+        "present": 1
+    }
+}

hal/api/mbed.h

@@ -26,6 +26,10 @@
 #include "network-socket/nsapi.h"
 #endif
 
+#if MBED_CONF_EVENTS_PRESENT
+#include "events/mbed_events.h"
+#endif
+
 #include "toolchain.h"
 #include "platform.h"