fix power button shutdown: use an orb topic instead of a work queue call

px4_shutdown_request() was called from the power button IRQ callback, which
invoked a work queue callback. But on NuttX, the work queue uses a
semaphore, and thus it cannot be called from IRQ context.
This patch switches to publishing an uORB msg instead, which is handled in
the commander main thread.

To increase failure resistance, we could subscribe to the same topic in
another module for redundancy, in case commander runs wild.

msg/CMakeLists.txt

@@ -81,6 +81,7 @@ set(msg_file_names
 	parameter_update.msg
 	position_setpoint.msg
 	position_setpoint_triplet.msg
+	power_button_state.msg
 	pwm_input.msg
 	qshell_req.msg
 	rc_channels.msg

msg/power_button_state.msg

@@ -0,0 +1,8 @@
+# power button state notification message
+
+uint8 PWR_BUTTON_STATE_IDEL = 0             # Button went up without meeting shutdown button down time (delete event)
+uint8 PWR_BUTTON_STATE_DOWN = 1             # Button went Down
+uint8 PWR_BUTTON_STATE_UP = 2               # Button went Up
+uint8 PWR_BUTTON_STATE_REQUEST_SHUTDOWN = 3 # Button went Up after meeting shutdown button down time
+
+uint8 event                                 # one of PWR_BUTTON_STATE_*