Remove the makefile based build system

[esden]

Detailed description

As we are already using meson for a while with pretty good success and the gnu makefile based system is starting to require maintenance effort. It is time to retire the makefiles.

If it turns out that we need to bring them back for some reason it should still be relatively easy to do.

Still this is an end of an era. :)

As a side-effect this PR also deletes the libopencm3 submodule as it is not needed any more.

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

[sidprice]

(Almost) End of an era! But what an era!