s390/boot: Improve decompression error reporting

Currently, decompression error messages can be very uninformative:
[    0.029853] startup: read error
[    0.040507] startup:  -- System halted

Improve these messages to make it clear that the error originates from
the decompression code. Additionally, on decompression failures, if
bootdebug is enabled, dump the message ring buffer before halting. This
provides more context for diagnosing startup issues.

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>