Add a mechanism for force building a particular community layout

[zvecr]

Description

I use a nori as a 4x12 keyboard and 4x4 macro pad, and have community layouts for both. Branch here

vagrant@vagrant:/vagrant$ ls layouts/community/*/zvecr
layouts/community/ortho_4x12/zvecr:
config.h  keymap.c  readme.md  rules.mk

layouts/community/ortho_4x4/zvecr:
config.h  keymap.c  readme.md  rules.mk

Currently there is no way to build both keymaps and make 40percentclub/nori:zvecr builds whatever it finds first, in this case ortho_4x12.

So far, i have added an additional flag that allows a particular keymap to be targeted

make 40percentclub/nori:zvecr FORCE_LAYOUT=ortho_4x4
make 40percentclub/nori:zvecr FORCE_LAYOUT=ortho_4x12

TBD

• What docs would need updating? done
• Should the filename contain the layout used? done
• Should this be extended to allow it to be specified in the keymap argument, eg
  make 40percentclub/nori:ortho_4x4/zvecr will not do for now, but could add in a future PR

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.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document. (https://docs.qmk.fm/#/contributing)
• 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).

[drashna]

Documentation about this should go here:
https://github.com/qmk/qmk_firmware/blob/master/docs/feature_layouts.md

And yeas, please do add documentation about this.

[mechmerlin]

Thanks!