px4io: support PX4IO_P_SETUP_FORCE_SAFETY_OFF

this allows the FMU to force the safety off on the IO board. Useful in
two cases:

 1) vehicles where the safety switch is impractical or not useful
 (eg. HAB planes or internal combustion motors)

 2) doing ESC calibration on multi-copters

src/modules/px4iofirmware/protocol.h

@@ -211,6 +211,10 @@ enum {							/* DSM bind states */
 						/* 12 occupied by CRC */
 #define PX4IO_P_SETUP_RC_THR_FAILSAFE_US	13	/**< the throttle failsafe pulse length in microseconds */
 
+#define PX4IO_P_SETUP_FORCE_SAFETY_OFF	       12	/* force safety switch into
+                                                           'armed' (PWM enabled) state */
+#define PX4IO_FORCE_SAFETY_MAGIC            22027       /* required argument for force safety (random) */
+
 /* autopilot control values, -10000..10000 */
 #define PX4IO_PAGE_CONTROLS			51		/**< actuator control groups, one after the other, 8 wide */
 #define PX4IO_P_CONTROLS_GROUP_0		(PX4IO_PROTOCOL_MAX_CONTROL_COUNT * 0)	/**< 0..PX4IO_PROTOCOL_MAX_CONTROL_COUNT - 1 */

src/modules/px4iofirmware/registers.c

@@ -570,6 +570,12 @@ registers_set_one(uint8_t page, uint8_t offset, uint16_t value)
 			dsm_bind(value & 0x0f, (value >> 4) & 0xF);
 			break;
 
+		case PX4IO_P_SETUP_FORCE_SAFETY_OFF:
+			if (value == PX4IO_FORCE_SAFETY_MAGIC) {
+				r_status_flags |= PX4IO_P_STATUS_FLAGS_SAFETY_OFF;
+			}
+			break;
+
 		default:
 			return -1;
 		}