posix rcS: use sed instead of grep -P

The -P argument seems not to be supported on OSX

ROMFS/px4fmu_common/init.d-posix/rcS

@@ -40,8 +40,8 @@ if [ "$PX4_SIM_MODEL" == "shell" ]; then
 	set RUN_MINIMAL_SHELL yes
 else
 	# Find the matching Autostart ID (file name has the form: [0-9]+_${PX4_SIM_MODEL})
-	# shellcheck disable=SC2010 #(the file names don't contain spaces)
-	REQUESTED_AUTOSTART=$(ls "$SCRIPT_DIR" | grep -oP '^[0-9]+(?=_'${PX4_SIM_MODEL}'$)')
+	# shellcheck disable=SC2012 #(the file names don't contain spaces)
+	REQUESTED_AUTOSTART=$(ls "$SCRIPT_DIR" | sed -n 's/^\([0-9][0-9]*\)_'${PX4_SIM_MODEL}'$/\1/p')
 	if [ -z "$REQUESTED_AUTOSTART" ]; then
 		echo "Error: Unknown model '$PX4_SIM_MODEL'"
 		exit -1