Since 1.31.0 cargo requires curl to support http2. Enabling http2 in
curl results in a dependency loop. This commit disables the use of
multiplexing and http2.
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.
Johan Anderholm
Hongxu Jia
Mike Yagley
Steven Walter
Doug Goldstein