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!

[9names]

I think I skimmed through the hacking guide but missed that section.
If the bootloader commands were in a code block like every other console command on that page, and used meson to invoke the command rather than cd'ing and then invoking ninja directly, maybe it would stand out more?

I can't find a repo for the firmware hacking guide to make a PR for that too, so here's what I suggest for the updated hacking guide text:

---

To start, we need to create a build directory from a configuration with Meson and then ask Meson to run the build:

meson setup build --cross-file=cross-file/native.ini
meson compile -C build

This will result in the following binary files in the build directory:

blackmagic_native_firmware.elf - ELF binary of the Black Magic debug probe.

blackmagic_native_firmware.bin - Flat binary of the Black Magic debug probe, load at 0x8002000.

If you need the bootloader as well, after running the above build run

meson compile -C build boot-bin

which will compile the bootloader and generate a .bin file for it for use with provisioning tools. This results in two files:

blackmagic_native_bootloader.elf - ELF binary of the Black Magic DFU bootloader.

blackmagic_native_bootloader.bin - Flat binary of the DFU bootloader, load at 0x8000000.