review samples

[barbieri]

We should review samples so they are uniform and up to date to newest practices, as well as adding a README.md file per sample describing what it is, how to use it.

Newest practices: https://github.com/solettaproject/soletta-riot-samples/blob/master/simple-fbp/main.c still refers directly to the node type symbols, instead of using the function. Also not sure what is good to have the main.c instead of main.fbp and call sol-fbp-generator.

Uniform: some samples use the recommended SOL_MAIN_DEFAULT(), while others use a traditional main.c.

Also plan in a way we can use the same samples in different small-oses, so we can rename the repository and use it for Contiki and Zephyr.