PAYG: Install PAYG components for EOS OSTree

[starnight]

Both EOS non-PAYG and PAYG system share the same OSTree branch. Which means they all have PAYG components. So, install eos-payg.bst and eos-payg-nonfree.bst directly. And, build eos-payg.bst and eos-payg-nonfree.bst by following current eos-payg & eos-payg-nonfree's build configuration.

Part-of: #165

[ssssam]

If we merge this change as-is, nobody outside Endless will ever be able to build eos-build-meta.git out of the box. As they will not have access to eos-payg-nonfree.git.

The reason for adding -o payg, which defaults to false for local builds but is set to true in CI, was to get the following developer experience:

• All CI builds of the ostree stage include PAYG, and can be triggered by anyone who has a pull request (I believe there's some security around triggering PRs for external contributors)
• Local builds of the ostree stage don't include PAYG by default, and can be run by anyone who follows the public docs
• Endless staff can follow special instructions to enable PAYG in their local ostree build

I didn't yet push the special instructions, maybe it makes sense for them to live outside eos-build-meta.git but I am not sure where.

If you think it's better to optimize the workflow for Endless staff and make it less convenient for 3rd parties to build eos-build-meta, then let's update the docs in eos-build-meta.git to:

• Advise building with -o payg false in case you don't have access to the private repos
• Document setting up .netrc in case you do

[starnight]

ah! Make sense. And, hope #179 ease the PAYG integration test.

I'll split the PAYG meson config as another PR.