Add option to compile blackpill-f4* with SHIELD option, using meson

[lenvm]

Detailed description

This PR adds back the option to compile using the SHIELD macro for blackpill-f4* boards.

This functionality was available when the code was compiled using make, but got lost in the translation to meson. This PR adds back this functionality.

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]

This LGTM, so merging. Thank you for the contribution!

[dragonmux]

Addendum: Please would you update your commit message to properly apply a prefix per the contrib guidelines - something like blackpill-f4/meson: in this case

[lenvm]

Sure! Commit message is updated!

[dragonmux]

Thank you kindly!