Use Alternate path in SenML payload.

[sbernard31]

This aims to implement #1438.

That means that when a device use an alternate path, it MUST be added to path in SenML (JSON or CBOR) payload.

So if alternate path is /lwm2m, it will looks like :

[
  { "n":"/lwm2m/3/0/0", "vs":"Open Mobile Alliance" }
]

Before this PR, it was :

[
  { "n":"/3/0/0", "vs":"Open Mobile Alliance" }
]

(This PR need probably some cleaning before to integrate in master)