s390/boot: Replace boot_printk() with loglevel-specific helpers

s390/boot: Replace boot_printk() with loglevel-specific helpers

Replaces boot_printk() calls with appropriate loglevel-specific helpers
such as boot_emerg(), boot_warn(), and boot_debug().

Using functions with explicit log levels improves log clarity and aligns
the boot code with standard kernel logging practices. This makes it
easier to filter and manage boot-time messages based on their severity.

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>