blackpill/platform: Correct the condiitonal compilation symbol to initialize the bootloader LED pin.

[sidprice]

This PR corrects a naming issue with the Blackpill-F4 platform.

The bootloader LED pin was not being setup.

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

None

[dragonmux]

LGTM, well spotted - merging. Thank you for the contribution!