Add RTX_LOG macro option to top-level rebar.config

[shino]

Before this commit, rebar3 release outputs relx's info log without line breaks.
This commit adds RTX_LOG macro to relx's erl_opts.

Steps to reproduce:

1. rebar3 new app foo
2. cd foo
3. Append {relx, [{release, {foo, "0.0.1"}, [kernel, sasl, stdlib, foo], []}]}.
4. rebar3 release

At the step 4, expectation is that it outputs (reasonably) concise log lines, but it outputs:

% rebar3.head release
===> Verifying dependencies...
===> Analyzing applications...
===> Compiling foo
Solving Release foo-0.0.1Resolved foo-0.0.1release: foo-0.0.1
     erts: 13.1.1
     goals:
          kernel
          sasl
          stdlib
          foo
     applications:
          {kernel,"8.5.1"}
          {stdlib,"4.1.1"}
          {sasl,"4.2"}
          {foo,"0.1.0"}
Assembling release foo-0.0.1...Release output dir /home/shino/g/rebar3/foo/_build/default/rel/fooRewriting .app file: /home/shino/g/rebar3/foo/_build/default/rel/foo/lib/kernel-8.5.1/ebin/kernel.appRewriting .app file: /home/shino/g/rebar3/foo/_build/default/rel/foo/lib/stdlib-4.1.1/ebin/stdlib.appRewriting .app file: /home/shino/g/rebar3/foo/_build/default/rel/foo/lib/sasl-4.2/ebin/sasl.appRewriting .app file: /home/shino/g/rebar3/foo/_build/default/rel/foo/lib/foo-0.1.0/ebin/foo.apprelease start script createdRelease successfully assembled: _build/default/rel/foo

[ferd]

Sorry for taking so long to get to this! Thanks a lot for the contribution!