Update_blackpill_readme

[sidprice]

This PR addresses some issues with the README file for the Blackpill platforms.

1 - The example scripts include a serial number for the Blackpill, this is not required, and if the given number is used, Blackpill will not be found. The serial number entry is removed from the example commands. 2 - The Make-based build system was recently removed from the repository and therefore, these instructions are removed. Leaving them in would cause failures when a user followed the given instructions. Particularly as they are at the start of the README file. 3 - The instructions for setup and compile used a folder named 'build'. Currently, the project uses build directories that reflect the name of the platform and platform-specific names for the binaries. This greatly reduces confusion about which binaries relate to which platform. Additionally, while the repository supports three variants of the Blackpill platform, all the example commands relate to just one platform. Additions were made to make note of the approach taken, and the commands converted to be 'generic.' 4 - A short note was added about the BMD bootloader and a link to the bmputil project. 5 - Finally, the order of the instructions was changed to reflect the Meson 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

None

[qwerty19106]

Note that we pass xxxxxx=blackpill-f411ce to meson setup for example. But we should pass xxxxxx = blackpill_f411ce to ninja: blackmagic_blackpill_f411ce_firmware.bin. - != _