Allow overriding the rule for $(KEYMAP_OUTPUT)/src/keymap.c

[pfn]

Description

Reorder the keymap.json branch in build_keyboard.mk such that
keymap rules.mk definitions can define their own rule for the
generated keymap.c and config.h files

This is also a step towards #6873; provides a similar solution to what #13197 did, but this time by allowing a keymap to create their own keymap.c generation override.

For example, with the changes included on this branch, I am able to do this:

$(KEYMAP_C): $(KEYMAP_JSON)
	$(QMK_BIN) json2c --quiet --output $(KEYMAP_C) $(KEYMAP_JSON)
	sed -i -e '2 i #include "pfn_keymap.h"' $(KEYMAP_C)

The downside of this approach is that there is an overridden recipe warning, but perhaps that is a good thing.

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

• Add custom keycode support to JSON files #6873

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).

[pfn]

Lots of push back on this change, so I have a workaround that works for this situation as well:

1. create a stub keymap.c, it can be empty or contain code as desired
2. download the configurator json into any arbitrarily named file.json, it must not be named keymap.json in the keymap folder
3. modify rules.mk in the keymap folder as follows

SRC += keymap_configurator.c

.SECONDEXPANSION # required to allow expanding $$(KEYMAP_PATH), $(KEYMAP_PATH) doesn't expand
keymap_configurator.c: $$(KEYMAP_PATH)/pfn.json
	$(QMK_BIN) json2c --quiet --output $@ $<
	sed -i -e '2 i #include "pfn_keymap.h"' $@