Move platform build scripts to packaging repo

[arobro]

Most platforms build scripts were moved to the packaging repo some years ago, with only the Windows platform remaining.

All remaining code in the platform directory has now been moved and merged into the packaging repo for consistency. (The cross compile scripts were also updated to work with the new directory structure and latest MythTV)

This commit removes the platform directory and therefore the corresponding duplicate code.

Merged packaging commit:
MythTV/packaging@a969adc

Thank you for contributing to MythTV!

Please review the checklist below to ensure that your contribution
to MythTV is ready for review by our developers.

It is helpful to regularly rebase your pull request to ensure changes
apply cleanly to the current MythTV development branch.

Checklist

• [ x] contribution does not duplicate one of our existing pull requests
• [ x] contribution is in a branch rebased against master
• [ x] code compiles successfully without errors
• [ x] code follows the MythTV Coding Standards
• [ x] documentation added/updated/removed where necessary
• [ x] commits are logically organised and have good commit messages