log: Allow LOG_DEBUG to always enable log output

At present if CONFIG_LOG enabled, putting LOG_DEBUG at the top of a file
(before log.h inclusion) causes _log() to be executed for every log()
call, regardless of the build- or run-time logging level.

However there is no guarantee that the log record will actually be
displayed. If the current log level is lower than LOGL_DEBUG then it will
not be.

Add a way to signal that the log record should always be displayed and
update log_passes_filters() to handle this.

Signed-off-by: Simon Glass <sjg@chromium.org>