[RFC] west: allow list of manifest files

[tejlmand]

Currently the west config manifest.file allows only a single file.

If multiple manifest files are required, then the user must manually create a custom manifest file and point manifest.file setting to the custom file, for example:

manifest:
  self:
    import:
    - fileA.yml
    - fileB.yml

This commit allows manifest.file to be a list so that the following west config manifest.file fileA.yml,fileB.yml will behave as the above manifest snippet.

, is used as separator, but is configurable using west config manifest.file-separator to support use-cases where manifest filename contains commas.