Support target overrides from config.toml or env variable

[krishnakumar4a4]

Add support for overriding target arch through config.toml, environment variable and a --config argument to cargo and make sure it works correctly with the --target-arch argument of cargo-wdk.

[gurry]

A fix for this will also fix https://github.com/microsoft/windows-drivers-rs/issues/89

[wmmc88]

A fix for this will also fix #89

Solving this issue doesn't necessarily close #89. #89 would also require all the packaging steps to support multiple targets at once

[gurry]

A fix for this will also fix #89

Solving this issue doesn't necessarily close #89. #89 would also require all the packaging steps to support multiple targets at once

If the requirement of #89 is to be able to build a single workspace that has projects with different targets, then that will be possible after https://github.com/microsoft/windows-drivers-rs/pull/494 because the packaging steps will also use the target that's specified in config.tom