feat(coreos-base/coreos): add mdadm

Total additional image size:

```
492K    /build/amd64-generic/sbin/mdadm
332K    /build/amd64-generic/sbin/mdmon
184K    /build/amd64-generic/sbin/mdassemble
```
Merge pull request #360 from philips/mdadm

feat(coreos-base/coreos): add mdadm