Create a build error if no bootloader is specified.

[tzarc]

Description

Some builds getting merged from master to develop don't include a BOOTLOADER in their rules.mk file -- bootmagic and the like don't work as a result. This flags any missing bootloader assignment as an error, preventing people flashing binaries that "aren't recoverable" without hardware intervention.

Types of Changes

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

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).