s390/boot: Dump message ring buffer on crash with bootdebug

Dump the boot message ring buffer when a crash occurs during boot, but
only if bootdebug is enabled. This helps assist in analyzing boot-time
issues by providing additional debugging information.

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>