smaccm-mavlink: generalize some types

* We can pass around a Ref to the tx sequence number, rather than a
  MemArea, due to the changes to addrOf a while back.

* beyond that: I think I just didn't understand the types well enough
  when I wrote this code the first time
smaccm-mavlink generator: packing function's type generalizes a bit
smaccm-mavlink: regenerate messages with type signature change
SMACCMPilot GCS Transmit: remove deprecated USART sender
SMACCMPilot GCS Transmit Task: redefined in terms of tower byte stream
SMACCMPilot main: fix the build
smaccmpilot: build stabilize app directly without intermediate libflight

* tower generates ISR handlers which, as weak symbols, need to be passed
  to the linker in objects rather than in libraries. So, we can't build
  tower code into an in intermediate library, we must build it into
  application code directly.

* may be time to refactor smaccmpilot into a separate tower app and tower
  compiler module, where the tower app is just library code. We'll worry
  about that separately.
disable external ardupilot code from the build

* we get all of the ardupilot components we need from the forked
  standalone_apahrs code in src/flight/.