Fix: platforms/blackpill-f4: invert state of SET_IDLE_STATE

[lenvm]

Detailed description

This pull request fixes the issue that LED_IDLE_RUN is not working on the blackpill-f4 platforms when it is assigned to PC13, the onboard LED. The issue is caused by the need to set PC13 to low to turn the LED on, rather than to set PC13 to high to turn the LED on.
The root cause of this is the schematic of how the LED is connected to PC13:
3.3v -> R 1k -> Blue LED -> PC13 (reference).

This pull request fixes the issue described, by inverting the state used by for SET_IDLE_STATE. This results in setting PC13 to low when the state is true, and PC13 to high when the state is false. This is exactly the behavior needed to turn the LED on when the state is true.

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 (make PROBE_HOST=native)
• It builds as BMDA (make PROBE_HOST=hosted)
• 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

This issue is a partial fix to #1516

[lenvm]

@dragonmux, as this is a fix for an existing issue with the blackpill-f4 platforms, can it also be included in the v1.10.0 release?

[dragonmux]

Good catch! Given how simple this fix is, and how low risk, we're going to merge this as part of v1.10.0.