Configurable Serial USART Timeout

[XScorpion2]

Description

Added an option to allow overriding the serial usart timeout. Reducing this on a STM32F303CC Split has shown a noticeable improvement on keyboard startup speed as well as a very small improvement on matrix scan speed. Going from 100ms to 10ms made it startup significantly faster (sadly didn't time it), and the scan speed improved by ~50 scans per second.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).