Because devtool inhibits all the normal bitbake fetchers, we can't mask crates.io.
Use EXTERNALSRC to detect if we're being built with devtool, and only mask
crates.io if we're not.
Fixes#195
rustbuild is a cargo based way to build Rust but all of our helper bits
in cargo.bbclass would create a chicken/egg issue with dependencies
since we pull down pre-built binaries and use those for boot strapping.