HoTT update.

There's a number of changes in this commit, all related.

Improved efficiency of preparing HoTT messages.

All HoTT message data is no-longer reset each time, going forward old
data is re-used unless it is updated.  This should help with GPS
co-ordinates too since they were erased from subsequent responses when
ideally the last-known co-ordinates should be transmitted each time.

The previous update was sending one too many bytes due to the
aligned/padded uint16_t values in the HoTT data structures.  There were
two solutions, use #pragma to pack the structure or avoid larger types
in the structures, the latter made sense and results in more portable
code since it's a wire format.

Updated HoTT structures to use latest message structure from Ardupilot.
Currently the variables are in underscore_format, that will likely
change in future.

Cleaned up and deleted old ported code that was #ifdef'd out.  Some test
code still remains for now.  This will be deleted in due course.

Introduced bitmask for EAM alarm flags.

Corrected documentation.