RTOS - add config mbed_lib.json file

Defines that rtos is present, this resolves in the macro:
"MBED_CONF_RTOS_PRESENT=1"

Author: 0xc0170

rtos/mbed_lib.json

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