doxygen: Add RTOS present macro

There is functionality hidden behind this macro. As result, doxygen won't include these. It should be
enabled to cover this functionality as well.

If we consider baremetal, each functionality not available contain own note about this.

Author: 0xc0170

doxyfile_options

@@ -2102,6 +2102,7 @@ PREDEFINED             = DOXYGEN_ONLY            \
                          DEVICE_QSPI             \
                          DEVICE_STORAGE          \
                          DEVICE_WATCHDOG         \
+                         MBED_CONF_RTOS_PRESENT  \
                          "TFM_LVL=1" \
                          "MBED_DEPRECATED_SINCE(d, m)=" \
                          "MBED_ENABLE_IF_CALLBACK_COMPATIBLE(F, M)=" \