There are 5 logging levels:
* ERROR
* WARN
* INFO
* DEBUG
* TRACE
There are 2 logging context:
* with
* without device
DEBUG and TRACE records are printed only if bootverbose.
Logging records are printed with source code line information if acceptable
logging level is DEBUG or TRACE.
Submitted by: Michael Zhilin <mizhka@gmail.com>
Differential Revision: https://reviews.freebsd.org/D6247
