pkg/libcsp: initial commit

[fjmolinas]

Contribution description

This PR adds libCSP as a package. This network stack was designed for embedded systems in smaller networks, such as Cubesats. More details on it can be found on its GitHub.

This work was started by @millotp while working with @ThingSat (CubeSat project with a RIOT-based payload in space). I'm adding this as a PR since it seems this network stack is used in CubeSat projects, so might be of interest for others.

Some things that could be improved:

• use of conn_can, although it makes for easy code leads IMO for duplicate stack usage, I think candev should probably be directly used.

Testing procedure

A test is provided with a README, an automatic test is included as well.

[fjmolinas]

@donsez might be interested.

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[Teufelchen1]

How can we move this forward? @fjmolinas still interested?