ci: switch to GitHub actions

[aparcar]

Instead of using a bunch of shell scripts this uses a single Dockerfile to handle both SDK and ImageBuilder container creation. The CI jobs are fairly straight forward with some shell hacks, however much simpler compare to the previous setup.

I'll get this in shape and merge it as some point - @ynezz objections?

Overal improvements:

• Much easier to run locally
• Uses buildbot container as a base instead of something selfbuild
• No deploy container required
• Automatic target detection based on openwrt.git

Missing:

• [x] GPG verification (required? uses signify already)
• [x] rootfs migration
• [ ] Push to hub.docker.com, too
• [x] Implement manual run for releases
• [x] Implement cronjob for snapshots
• [ ] Use CDN?
• [ ] Automated testing prior to pushing
• [ ] more?

[aparcar]

@ynezz if you find the time, could you please review this? The setup is much simpler.

[ynezz]

No objections, thanks!