Add a yotta config dependency for nanostack

[bremoran]

This is a backwards-compatible change for selectively importing nanostack. Nanostack can be imported into SAL by specifying the following config in either target.json or the application's config.json:

"mbed-os" : {
  "net" : {
    "stacks" : {
      "nanostack" : true
    }
  }
}

If nanostack has additional configuration, it can be specified in a dictionary instead of "true." This allows users to select nanostack or lwip as needed, at application level without specifying dependencies on either.

[bremoran]

cc @artokin @SeppoTakalo @bogdanm @autopulated

[autopulated]

:thumbsup:

[bogdanm]

+1