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.
TSC21
Beat Küng