s390/zcore: Rename ipl_block to mitigate name collision

With git commit 1e941d39
"s390: move ipl block to .boot.preserved.data section" the earl_ipl_block
got renamed to ipl_block and became publicly available via boot_data.h.
This might cause problems with zcore, which has it's own ipl_block
variable. Thus rename the ipl_block in zcore to prevent name collision
and highlight that it's only used locally.

Signed-off-by: Philipp Rudo <prudo@linux.ibm.com>
Fixes: 1e941d39 ("s390: move ipl block to .boot.preserved.data section")
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>