Add script that generates rules.mk files from Rust crates.
Based on and inspired by cargo2android.py script to generate
blueprint files for the Android build system.
The script has only been tested on the vendored crates used to
build Trusty (see external/rust/crates) and does not support
the full range of crate types or cargo features.
Bug: 281857510
Change-Id: I1951f75476c36b9a1b794c4a51483f2c7387b1f5
