Feature/misc meson improvements

[perigoso]

Detailed description

This is a minor cleanup/simplification of the meson configuration around the target enabling

This should result in no behavior changes, except for a small, but very intentional one.

Right now when manually configuring the build, i.e. not using the pre-made cross-files for the probes, the default behaviour was to enable all targets by default, resulting in the need to safe-guard against special cases for certain probes to reduce user confusion.

This changes things such that by default when manually configuring the build no targets are enabled by default, a helpful error message should be displayed about it (improvement over the previous compilation error).

I think this makes sense because this is to me an "advanced user" usage and reduces the maintenance burden on the build system.

Your checklist for this pull request

• [x] I've read the Code of Conduct
• [x] I've read the guidelines for contributing to this repository
• [x] It builds for hardware native (see Building the firmware)
• [x] It builds as BMDA (see Building the BMDA)
• [x] I've tested it to the best of my ability
• [x] My commit messages provide a useful short description of what the commits do

Closing issues

[perigoso]

Whoops, i missed that this needed rebasing