Add overrides for Makefile syntax highlighting

[rbreslow]

Fix GitHub syntax highlighting for the Coreboot/Heads config files, and modules, which are all Makefiles.

See: https://github.com/github/linguist/blob/master/docs/overrides.md

[rbreslow]

@tlaurion Looks like tests failed but it's not immediately clear why: https://app.circleci.com/pipelines/github/rbreslow/heads/2/workflows/33fdc170-1564-44e7-8554-d6914f53b896/jobs/. Have you seen this before?

[tlaurion]

@tlaurion Looks like tests failed but it's not immediately clear why: https://app.circleci.com/pipelines/github/rbreslow/heads/2/workflows/33fdc170-1564-44e7-8554-d6914f53b896/jobs/. Have you seen this before?

@rbreslow Can't access that URL, but saw that postcar fails for what seems another race condition when building with multiple threads under https://app.circleci.com/pipelines/github/rbreslow/heads/4/workflows/8972250e-c1fb-4595-afdd-7f2c5d6e2b45/jobs/70?invite=true#step-106-635

Hopefully this is transcient and asking CircleCI to build from failed step resolves the issue, otherwise more investigation will be needed....

For the sake of making sure master still works without cache, I have restarted a fresh build (without cache: that is under CircleCI's project settings -> Environement Variables -> Add Environment Variable (CACHE_VERSION where you pick a unique string that wasn't used as part of cache name for the last 30 days (I pick dates like 2023-01-18 or name of PR etc)

Fresh build of master is happening under https://app.circleci.com/pipelines/github/osresearch/heads/500/workflows/df8f322d-79fb-4460-962b-c39b6f747984 and I will check for failing....

[tlaurion]

@rbreslow : Sidenote, I see here that CircleCI is now at librem_14 step, so you might not have to change variable name.
You will create 3 cache levels after success of all boards being build, as described :

• Files taken into consideration to create digests part of 3 cache layers names: https://github.com/osresearch/heads/blob/e45980d33f46c9d3aa2855e1876e915a8fc53cdd/.circleci/config.yml#L63-L76
• Cache restoration happening early prior of board builds: https://github.com/osresearch/heads/blob/e45980d33f46c9d3aa2855e1876e915a8fc53cdd/.circleci/config.yml#L78-L88
• On top of that is persist to workspace logic, passing all build cache and directories pertaining to same coreboot version for builds, where we try to keep boards building for same kernel versions dependent on each other.

Idea here is that most changes under Heads are related to scripts being packed under heads cpio of initrd, where tools.cpio and modules.cpio barely change. Some more work is needed either into creating a proper docker image containing first layers of the caches so that we do not have to rebuild musl-cross-make that barely ever change, and keep coreboot cache that doesn;t change for months, while tools modules being packed also rarely change but when needed. But unfortunately as of now, persisting to workspace is still taking a lot of time to unpack and merge layers prior of saving cache, so this is still suboptimal (takes more then an hour to rebuild with cache because of the unpacking and persisting to workspace logic).