[FEATURE] Add support for custom bakery url

[darkspadez]

[FEATURE] Add support for custom bakery url

Currently hardcoded to extensions.flatcar.org/extensions/, though forks and individual bakerys would need to edit files directly to use their own.

How to use

Add --bakery <url> to bakery.sh during build time or --bakery <url> to tools/bake_flatcar_image.sh . This makes it easier for forks and other "bakerys" to specify their url instead of editing the files directly.

[t-lo]

We need to take care of this one as well, it is used to query version information of existing images from github to determine which ones to build.

[t-lo]

Thank you for your contribution! May I suggest putting both bakery= and bakery_url into a separate env file which is sourced by lib/libbakery if it exists, to overwrite the defaults? This would make a fork work more flexibly as there only needs to be one change on top (the custom env file) - which is easy to rebase.

[darkspadez]

@t-lo Just to confirm, switch from --bakery argument to an environment file?

[t-lo]

@t-lo Just to confirm, switch from --bakery argument to an environment file?

@darkspadez Yes, an optional one. I.e. leave defaults in lib/libbakery.sh as is but source a custom env file after that if that env file exists.

This should make it easier for forks to customise the builds. I'm thinking rebases and merge conflicts here: when customisations happen in a separate file that doesn't exist in the upstream bakery, we effectively avoid merge conflicts during rebases / fast-forwards.