Fix broken bootloader builds in develop.

[daskygit]

Description

Most are just missing a bootloader assignment as the change to info.py in #15638 seems to now require the bootloader to be set specifically in info.json.

This also adds a weak enter_bootloader_mode_if_requested so chibios builds with an unknown bootloader will compile successfully with an accompanying pragma message.

These keychron boards require the BOOTLOADER setting and STM32_BOOTLOADER_ADDRESS defining (which would go against the previous work if added to rules.mk). Maybe the MCU configuration should be added to builddefs/mcu_selection.mk in a separate PR. (Compiles as a unknown bootloader with this PR)

  keychron/q2/rev_0110:default
  keychron/q2/rev_0110:via
  keychron/q2/rev_0111:default
  keychron/q2/rev_0111:via
  keychron/q2/rev_0112:default
  keychron/q2/rev_0112:via
  keychron/q2/rev_0113:default
  keychron/q2/rev_0113:via

These 2 matrix boards using custom booloaders now also compiles with the unknown bootloader pragma message.

  matrix/m20add:default
  matrix/noah:default

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).

[daskygit]

Ta for the quick reviews.