config.toml now accepts a target.$TARGET.musl-root key that lets you
override the "build" musl-root value, which is set via the --musl-root
flag or via the build.musl-root key.

With this change, it's now possible to compile std for several musl
targets at once. Here's are the sample commands to do such thing:

```
$ configure \
    --enable-rustbuild \
    --target=x86_64-unknown-linux-musl,arm-unknown-linux-musleabi \
    --musl-root=/musl/x86_64-unknown-linux-musl/

$ edit config.toml && tail config.toml
[target.arm-unknown-linux-musleabi]
musl-root = "/x-tools/arm-unknown-linux-musleabi/arm-unknown-linux-musleabi/sysroot/usr"

$ make
```