Move bootloader.mk to platforms

[fauxpark]

Description

At this point every keyboard should have either a valid bootloader set, or custom. Anything else should generate an error.

This change splits up bootloader.mk into platform-specific makefiles, to further lock out anything that is invalid (eg. md-boot for atmega32u4). This would fail to compile anyway as there is no md-boot.c under the AVR bootloaders directory, but now it should spit out a more user friendly error.

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).

[fauxpark]

 ☒ handwired/pytest/has_community: Can't find default keymap.

This lint error is unrelated. This board indeed has no keymaps at all; the intention I guess is to test community layout support. Not sure whether anything can/should be done about it.

[KarlK90]

LGTM!