SITL: handle 'make posix_sitl_shell' via unified rcS

The command is now: 'make posix none_shell'
which means: don't start a simulator and run a minimal shell only.

ROMFS/px4fmu_common/init.d-posix/rcS

@@ -14,14 +14,13 @@ then
 	exit 0
 fi
 
-
-# TODO: In the future we want to share rc.autostart with NuttX
-#source rc.autostart
+RUN_MINIMAL_SHELL=no
 
 # Use the variable set by sitl_run.sh to choose the model settings.
-if [ "$PX4_SIM_MODEL" == "iris" ]; then
+if [ "$PX4_SIM_MODEL" == "shell" ]; then
+	set RUN_MINIMAL_SHELL yes
+elif [ "$PX4_SIM_MODEL" == "iris" ]; then
 	set REQUESTED_AUTOSTART 10016
-
 elif [ "$PX4_SIM_MODEL" == "typhoon_h480" ]; then
 	set REQUESTED_AUTOSTART 6011
 else
@@ -35,6 +34,9 @@ then
 	param load
 fi
 
+# exit early when the minimal shell is requested
+[ $RUN_MINIMAL_SHELL == yes ] && exit 0
+
 if param compare SYS_AUTOSTART $REQUESTED_AUTOSTART
 then
 	set AUTOCNF no