List of source files as option to compilation

[jenia81]

Currently, mbed-cli compile command requires list of folders with source files.
It would be a benefit to have an option that enables to compile source files from a list.

In our provisioning tree(that we currently compile with mbed-cli) we have folders that contains both library sources and tests/application sources.
For library compilation, we use --source option + .mbedignore for ignoring all tests/application sources, but for tests/application compilation we need to copy all relevant files outside and compile it from there.

It's a disadvantage of mbed-cli that I am sure will disturb every customer that has an existing tree and doesn't write code from scratch for mbed-os

[jenia81]

@trianglee
@mottigondabi