[FR] Provide well known path for including a local Makefile

[brianredbeard]

Description

Provide a set of well known file globs/paths for users to incorporate their own build logic without having to worry about merge conflicts or breaking changes.

Background

There are a few concurrent changes happening with relation to how Marlin is developed and tested. Since the 1.0.x branch I've used a Makefile in the root of the project for the management of the project. Moving to the 2.0.x branch required a little bit of re-tooling and incorporating PlatformIO commands instead of my previous avr-gcc commands, but this was not a problem as it was a one time change.

With the work being done on #19640 I may consider swapping some of these back out, though the library management of PlatformIO works well enough and does provide a bit of time savings.

Unfortunately #19981 caught me by surprise as there has never been a Makefile in the root of the project (at least as best I could tell checking all the way back to @boelle tagging 1.0.1 in 2014). Furthermore, the mutations it caused (noted in #20433) added additional annoyance, beyond the simple resolution of merge conflicts.

Personally, it would be nice to have some way of defining my own Makefile.local.mk type file in which I know I can add my own targets while being able to cleanly fast-forward the rest of the repository.

Sample change

I have a mock up in my current repository, which can be seen here:

https://github.com/brianredbeard/marlin-firmware/blob/45faff56f03c219233a08033806ecadcb233b053/Makefile#L1-L16

THIS_FILE := $(lastword $(MAKEFILE_LIST))
EXTRA_MAKEFILES := $(strip $(wildcard Makefile.*.mk))

ifneq ($(EXTRA_MAKEFILES),)
  contents := $(shell echo including extra makefiles $(EXTRA_MAKEFILES))
  include $(EXTRA_MAKEFILES)
endif

list:
	@$(MAKE) -pRrq -f $(THIS_FILE) : 2>/dev/null | awk -v RS= -F: '/^# File/,/^# Finished Make data base/ {if ($$1 !~ "^[#.]") {print $$1}}' | sort | egrep -v -e '^[^[:alnum:]]' -e '^$@$$'
.PHONY: list

In this change set I am establishing a conditional include (in the event that the files exist) of any files matching the pattern Makefile.*.mk will be included into the workflow. Additionally, a new target (list) is added which will enumerate all targets found across the composed Makefile.

Next steps

Discussion of the idea, specifically around: pros, cons, potential areas where this may break or introduce additional problems, etc.