s390: clarify compressed image code path

The way the decompressor is hooked into the start-up code is rather
subtle, with a mix of multiply-defined symbols and hardcoded address
literals. Add some comments at the junction points to clarify how it
works.

Signed-off-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>