OTP-21: New sys_config_src option in relx

With the release of OTP-21 and rebar3 3.6.0, which includes a new relx 3.25.0, there is better support for dynamic configuration of releases at runtime. If you are new to this concept checkout the rebar3 docs first

No longer will you be forced to declare RELX_REPLACE_OS_VARS and convert strings to integers in your code because the sys.config file had to be valid Erlang terms when building the release.

Instead, you can use {sys_config_src, "config/sys.config.src"} and {vm_args_src, "config/vm.args.src"} and the extended start up script will replace variables for .src files without the need to set RELX_REPLACE_OS_VARS.

Since this is based on a change in systools to support including sys.config.src without requiring validation of the contents you can also freely declare a config file that looks like this:

[{myapp, [{port, ${PORT}}]}].

An error, because this isn't a valid Erlang term file, will still be returned if your release configuration uses the sys_config option, but with sys_config_src it does not have to be valid terms until after variable replacement. In this case, PORT=8080 bin/myrel console will use a sys.config with contents [{myapp, [{port, 8080}]}].

vm_args_src was added to relx for consistency. The only thing that changes is how the start script handles it internally and that RELX_REPLACE_OS_VARS is not required for replacement to be run.

This change also will resolve at least a couple annoyances related to how relx had to handle sys.config before, https://github.com/erlang/rebar3/issues/1753 and https://github.com/erlware/relx/issues/604

OTP-21: New sys_config_src option in relx
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