Assemble the actual Suunto serial number

It turns out that the serial number returned by libdivecomputer isn't
really the serial number as interpreted by the vendor. Those tend to be
strings, but libdivecomputer gives us a 32bit number.

Some experimenting showed that for the Suunto devies tested the serial
number is encoded in that 32bit number:

It so happens that the Suunto serial number strings are strings that have
all numbers, but they aren't *one* number. They are four bytes
representing two numbers each, and the "23500027" string is actually the
four bytes 23 50 00 27 (0x17 0x32 0x00 0x1b). And libdivecomputer has
incorrectly parsed those four bytes as one number, not as the encoded
serial number string it is. So the value 389152795 is actually hex
0x1732001b, which is 0x17 0x32 0x00 0x1b, which is - 23 50 00 27.

This should be done by libdivecomputer, but hey, in the meantime this at
least shows the concept. And helps test the XML save/restore code.

It depends on the two patches that create the whole "device.c"
infrastructure, of course. With this, my dive file ends up having the
settings section look like this:

  <divecomputerid model='Suunto Vyper Air' deviceid='d4629110'
serial='01201094' firmware='1.1.22'/>
  <divecomputerid model='Suunto HelO2' deviceid='995dd566'
serial='23500027' firmware='1.0.4'/>

where the format of the firmware version is something I guessed at,
but it was the obvious choice (again, it's byte-based, I'm ignoring
the high byte that is zero for both of my Suuntos).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>