Note that multiple operating systems booting on a single machine is still something of a black art. This document does not even attempt to document the various boot managers, which vary by architecture and even by subarchitecture. You should see your boot manager's documentation for more information.
The boot loader on S/390 is „zipl“. ZIPL is similar in configuration and usage to LILO, with a few exceptions. Please take a look at „LINUX for S/390 Device Drivers and Installation Commands“ from IBM's developerWorks web site if you want to know more about ZIPL.