ROMFS: Switch HIL to a setting orthogonal to airframes

This makes it easy to flip any airframe config over to HIL.

ROMFS/px4fmu_common/init.d/1000_rc_fw_easystar.hil

@@ -44,5 +44,5 @@ then
 	param set RWTO_TKOFF 1
 fi
 
-set HIL yes
+param set SYS_HITL 1
 set MIXER AERT

ROMFS/px4fmu_common/init.d/1001_rc_quad_x.hil

@@ -11,5 +11,4 @@
 sh /etc/init.d/rc.mc_defaults
 
 set MIXER quad_x
-
-set HIL yes
+param set SYS_HITL 1

ROMFS/px4fmu_common/init.d/rcS

@@ -204,6 +204,12 @@ then
 		set FMU_ARGS "-t"
 	fi
 
+	if param compare SYS_HITL 1
+	then
+		# Enable HITL mode
+		set HIL yes
+	fi
+
 	#
 	# Set USE_IO flag
 	#