misc: Add bootloader build instructions to README.md

[9names]

Detailed description

Add basic instructions on how to build the BMP bootloader to README.md.
The existing documentation tells you how to build probe firmware but assumes you've already got a working bootloader - took me a while to work out how to build this, so I figure it is worth documenting.

Your checklist for this pull request

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

Closing issues

[dragonmux]

We do actually document this on the website in the firmware hacking guide, but having it more readily discoverable here and in such a cleanly described way seems reasonable to us, so LGTM - merging.

Thank you for the contribution!