[S390] New header file ipl.h

Setup.h has been misused for ipl related stuff in the past. We now move
everything, which has to do with ipl and reipl to a new header file named
"ipl.h".

Signed-off-by: Michael Holzheu <holzheu@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>