Host packages and merging Docker images

[matteodelabre]

(I’m using target to refer to the reMarkable system and host to refer to the containerized build environment—see #22)

Some packages, when compiled, provide binaries to be used in the build context of other recipes. Such binaries are built for the host architecture instead of the target architecture. OpenWrt refers to these as host tools (see their docs). For example, qtbase provides qmake, that is used to build Qt-dependent projects. We should provide a way for recipes to export some of their artifacts as “host packages”, and a way for other recipes to reuse those artifacts as part of their build process. We should make sure such packages are not installable on the target.

We could leverage this feature to merge all of our Docker images (qt, rust, python, golang) into a single one, by moving the different build tools provided by each of them to separate packages that can be required by individual recipes, and only leaving the base cross-compiling toolchain in the image. This would also make it easier to compose several tools together (what if we need to compile something that needs both Qt and Go? currently, we’d need to create a custom Docker image, see https://blog.redteam-pentesting.de/2021/remarkable-encryption/#developing-the-passphrase-prompt and https://github.com/RedTeamPentesting/remarkable-encryption).