cmd/build-cored-release: new command
cmd/build-cored-release: new command

build-cored-release compiles or cross-compiles cored and corectl
at the specified ref. Its main use case is within automated build
scripts that generate installer packages.

Usage is as follows:

```
build-cored-release <releaseRef> <outputPath>
```

- releaseRef must be a valid git ref in the locally-cloned repo.
- outputPath must be a valid absolute or relative directory.

The buildTag build variable is set to releaseRef.

Optional parameters are specified via environment variables:

- GOOS
- GOARCH
- DATE: a value for the buildDate build variable. By default, the current
time
  is used.

Closes #275

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain