package: add libmbus library

libmbus is an open source M-bus (Meter-Bus) library.

The Meter-Bus is a standard for reading out meter data from electricity
meters, heat meters, gas meters, etc.

[Peter: minor whitespace fixes]
Signed-off-by: Sagaert Johan <sagaert.johan@skynet.be>
Reviewed-by: Luca <Ceresoli&lt;luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>