We can now use 'make -j 32' again. All tests pass. There might
be some issues left with the areas that are not fully tested
yet (some plugins).

A few changes in behavior:

* 'make app' will also do 'make deps' unless SKIP_DEPS is set.
  You can use 'make app-build' instead to just build the application.

* 'make rel' will also do 'make app' (and therefore deps). There
  are no alternative target to keep the old behavior at this time.

If the local build.config has changes compared to upstream's
master branch, and $(ERLANG_MK_COMMIT) also brings changes to this
file, copying the local file to the checkout before switching to
$(ERLANG_MK_COMMIT) caused git-checkout(1) to abort with:

    error: Your local changes to the following files would be overwritten by checkout:
    	build.config
    Please, commit your changes or stash them before you can switch branches.
    Aborting

While here, fix `core-upgrade-custom-repo` test case: without switching
back the alternate erlang.mk repository to its master branch, the clone
always got the test-copyright branch by default, making the
ERLANG_MK_COMMIT check ineffective.

The `-n` flag is not standard and not portable. It is supported by many
Bourne shell to suppress the trailing newline character. But some old
shells do not support this and "-n" is printed.

This is the case for instance on some old versions of Mac OS X where
`/bin/sh` is Bash 3.2.

Main test Makefile expects "c_src" but for convenience I also
allow "c-src" and use that one as the prefix for the tests.

Pushing this now so I can figure out Windows.