OSTC firmware update prompt: use the stable changelog files

Heinrichs Weikamp is giving us stable URLs from which we can get the
latest stable version. The parsing is a bit simplistic, but it seems to
work.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Automatic OSTC firmware update: download file and try to install

This successfully downloads the hex file.
The actuall update of the OSTC fails before it gets started with a crash
when the ConfigureDiveComputer object is created.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>