Build Mbed application with out-of-tree modules

**Is your feature request related to a problem? Please describe.**
My Mbed project has a specific source tree:

> mbed_app/
> ├─ mbed_app.json
> third_party/
> ├─ wifi_lib/
> │ ├─ mbed_lib.json

I want to build a Mbed application with out-of-tree WiFi module (Mbed OS compatible [wifi-ism43362](https://github.com/ARMmbed/wifi-ism43362) ) but I do not see the possibility of indicating the path to it and join the configuration from mbed_lib.json.
Now, the only option is to move wifi_lib to mbed_project directory (create a symlink for example) and I would like to avoid it.

**Describe the solution you'd like**
Added option to include .json out-of-tree configuration file when generating mbed config.cmake. For example, this could be a new argument to the _mbed-tools configure_ command.

Start at the mbed-tools configure command and the code that generates config.cmake; trace how mbed_lib.json files are discovered and merged. Done means an out-of-tree configuration path can be supplied without moving or symlinking the module, and the generated configuration includes it.

Written by the indexing model from the issue text.