The logbook ringbuffer is now considered empty if any of the pointers is
outside the valid ringbuffer area. Compared to checking only against a
special empty value, this approach makes the code more robust against
invalid pointers.
Introducing a common base class allows to share more code between the
backends. Sharing the fingerprint data eliminates the need to pass it
with a function parameter.
The memory layout of all Oceanic devices is very similar, which allows
to share the parsing code between the different backends. Differences in
the layout are passed by means of a new layout descriptor structure.
Memory buffers are now allocated dynamically to support devices with
different amounts of memory.
