Adding cross-compilation profile

The cross-compilation profile works on a simple basis:

- the target system where the code will run has a pre-built Erlang
  runtime installed with SSL/TLS support (eg. `sudo apt-get install
  erlang-nox` for a wx-free Erlang on a debian or ubuntu host) only
- the host where the build is made has a compatible erlang version used
- the build host calls `rebar3 as portable do release, tar`
- the tarball at `_build/portable/rel/revault/revault-$VSN.tar.gz` is
  moved to the target host
- the target host unpacks the tarball with `tar -xvf revault-$VSN.tar.gz`
- the target host gets the proper TOML configuration and file layout
- the target host calls ./bin/revault console

The target host then runs the software and is possible to contact for
sync jobs.

This is not the same as a fully portable build that would be
self-contained, but it is much simpler to put in place than having to
set up a whole build environment for each target system, and will work
so long as none of the non-system libraries used require NIFs.

rebar.config

@@ -64,6 +64,16 @@
         {escript_name, revault_cli},
         {escript_emu_args, "%%! -name cli -setcookie revault_cookie +sbtu +A1\n"}
     ]},
+    {portable, [
+        %% meant to be run on a pre-built Erlang system on any supported OS
+        {relx, [
+            {include_src, false},
+            {debug_info, keep},
+            {dev_mode, false},
+            {include_erts, false},
+            {system_libs, false}
+        ]}
+    ]},
     {debug, [
         %% generate debug traces in gen_* processes
         {erl_opts, [{d, 'TEST'}]}