s390/boot: Add prefix filtering to bootdebug messages

Enhance boot debugging by allowing the "bootdebug" kernel parameter to
accept an optional comma-separated list of prefixes. Only debug messages
starting with these prefixes will be printed during boot. For example:

    bootdebug=startup,vmem

Not specifying a filter for the "bootdebug" parameter prints all debug
messages. The `boot_fmt` macro can be defined to set a common prefix:

    #define boot_fmt(fmt) "startup: " fmt

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>