add backtrace script and improve config file instructions

scripts/backtrace.sh allows the user to copy a Rust backtrace from Redox and retrieve the symbols.

• On Redox, set RUST_BACKTRACE=full and capture the resulting backtrace (usually copy/paste from the console)
• Save the backtrace in a file
• Run scripts/backtrace.sh with appropriate options to get a backtrace with function names and line numbers

Also, there are some comments and empty section headers added to the platform-specific config files, to help developers who want to copy and add to them.