include --releasever=/ in mkimage-yum.sh

This script failed when trying to create a docker image based on centos 7. Adding "--releasever=/" fixed it. According to yum man page:

```
Note: You may also want to use the option --releasever=/ when creating the installroot as otherwise the $releasever value is  taken  from  the  rpmdb  within  the installroot (and thus. will be empty, before creation).
```