ttyprintk: make the printk log level configurable

For some use cases it is handy to use a different printk log level than the
default (info) for the messages written to ttyprintk, so add a Kconfig
option similar to what we have for default console loglevel.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>