log: allow for message continuation

Some drivers use macro pr_cont() for continuing a message sent via printk.
Hence if we want to convert printk messaging to using the logging system,
we must support continuation of log messages too.

As pr_cont() does not provide a message level we need a means of
remembering the last log level.

With the patch a pseudo log level LOGL_CONT as well as a pseudo log
category LOGC_CONT are introduced. Using these results in the application
of the same log level and category as in the previous log message.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>