Merge branch 'ab/sha-makefile-doc'

Makefile comments updates and reordering to clarify knobs used to
choose SHA implementations.

* ab/sha-makefile-doc:
  Makefile: discuss SHAttered in *_SHA{1,256} discussion
  Makefile: document default SHA-1 backend on OSX
  Makefile & test-tool: replace "DC_SHA1" variable with a "define"
  Makefile: document SHA-1 and SHA-256 default and selection order
  Makefile: document default SHA-256 backend
  Makefile: rephrase the discussion of *_SHA1 knobs
  Makefile: create and use sections for "define" flag listing
  Makefile: correct DC_SHA1 documentation
  INSTALL: remove discussion of SHA-1 backends
  Makefile: always (re)set DC_SHA1 on fallback