renamed "args.toolchains" to "args.toolchain" because that looks better in the
help output:

```
./mpsd-software.py install --help
usage: mpsd-software.py install [-h] [--enable-build-cache] release toolchain [toolchain ...]
```

is better than

```
./mpsd-software.py install --help
usage: mpsd-software.py install [-h] [--enable-build-cache] release toolchains [toolchains ...]
```