s390/boot: Add bootdebug option to control debug messages

Suppress decompressor debug messages by default, similar to regular
kernel debug messages that require 'DEBUG' or 'dyndbg' to be enabled
(depending on CONFIG_DYNAMIC_DEBUG). Introduce a 'bootdebug' option to
enable printing these messages when needed.

All messages are still stored in the boot ring buffer regardless.

To enable boot debug messages:

  bootdebug debug

Or combine with 'earlyprintk' to print them without delay:

  bootdebug debug earlyprintk

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>