RTOS: Rework directory structure

Move system implementation to rtos/kernel directory to avoid confusion and make
navigation easier.

Author: bulislaw

doxyfile_options

@@ -840,7 +840,7 @@ EXCLUDE_PATTERNS       = */tools/* \
                          */TESTS/* \
                          */targets/* \
                          */BUILD/* \
-                         */rtos/rtx* \
+                         */rtos/kernel/* \
                          */cmsis/* \
                          */FEATURE_* \
                          */features/mbedtls/* \

doxygen_options.json

@@ -8,5 +8,5 @@
     "PREDEFINED": "DOXYGEN_ONLY \"MBED_DEPRECATED_SINCE(f, g)=\" \"MBED_ENABLE_IF_CALLBACK_COMPATIBLE(F, M)=\"",
     "EXPAND_AS_DEFINED": "", 
     "SKIP_FUNCTION_MACROS": "NO",
-    "EXCLUDE_PATTERNS": "*/tools/* */TESTS/* */targets/* */FEATURE_*/* */features/mbedtls/* */features/storage/* */features/unsupported/* */features/filesystem/* */BUILD/* */rtos/rtx*/* */cmsis/* */features/FEATURES_*"
+    "EXCLUDE_PATTERNS": "*/tools/* */TESTS/* */targets/* */FEATURE_*/* */features/mbedtls/* */features/storage/* */features/unsupported/* */features/filesystem/* */BUILD/* */rtos/kernel/* */cmsis/* */features/FEATURES_*"
 }