Fixes for bootloader refactor build failures

[fauxpark]

Description

#15450 broke compilation for some boards, and exposed a couple of problems with others.

• Moved STM32_BOOTLOADER_ADDRESS declarations to mcu_selection.mk; deleted all bootloader_defs.h and removed any instances of it as a define
• Removed reference to enter_bootloader_mode_if_requested() for matrix/abelx
• Set correct bootloader for mechmini/v1
• Set bootloader for mode/m65s
• Removed some logic in CLI that was causing the bootloader selection to default to stm32-dfu
• Added missing include for GD32 bootloader implementation

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).