Revert "Refactor makefiles"

[edwinb]

Reverts edwinb/Idris2#370

This turned out to cause a few problems, and I can't immediately see how to fix them so I'll need to revert for now. They are:

• In the top level config.mk, setting CC ?= clang doesn't do anything, since CC is set by default, so it should probably be :=
• Even if OPT isn't set, it builds idris2.c with -O2 due to a setting in dist/Makefile. Ideally, by default, the code in dist/rts should be built with -O2 since its stable rts code, but during development we don't want -O2 for the generated idris2.c

• Most worryingly, when I do make -C dist/rts clean I get an error:

  make: Entering directory '/home/edwin/Research/Yaffle/dist/rts'
  ../config.mk:1: /config.mk: No such file or directory
  make: *** No rule to make target '/config.mk'. Stop.

  I'm no Makefile expert, as you can probably tell, so I'd rather not get tied up in knots trying to work this one out.

[edwinb]

So much for reverting being simple. I guess I'll leave it for now.

[ska80]

• Even if OPT isn't set, it builds idris2.c with -O2 due to a setting in dist/Makefile. Ideally, by default, the code in dist/rts should be built with -O2 since its stable rts code, but during development we don't want -O2 for the generated idris2.c
• Most worryingly, when I do make -C dist/rts clean I get an error:

  make: Entering directory '/home/edwin/Research/Yaffle/dist/rts'
  ../config.mk:1: /config.mk: No such file or directory
  make: *** No rule to make target '/config.mk'. Stop.

@edwinb I have fixed those issues in PR #383

Now make -C dist/rts clean works as expected.