s390/boot: Add startup debugging support

Add boot_debug() calls to log various memory layout decisions and
randomization details during early startup, improving debugging
capabilities.

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>