libmodbus: add support for the libmodbus development release

libmodbus has two release streams:
- stable: which currently is 3.0.x and does only receive fixes
- development: 3.1.x which receives new features and according to upstream is also very stable

Some users require access to the newer features.
So a recipe for the development branch but leave the stable branch the default.
This is similar to what is done for the nginx recipe which also has two release streams.

Signed-off-by: Pascal Bach <pascal.bach@siemens.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>