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>

doc/develop/logging.rst

@@ -38,6 +38,9 @@ There are a number logging levels available, in increasing order of verbosity:
 * LOGL_DEBUG_CONTENT - Debug message showing full message content
 * LOGL_DEBUG_IO - Debug message showing hardware I/O access
 
+To continue a log message in a separate call of function log() use
+
+* LOGL_CONT - Use same log level as in previous call
 
 Logging category
 ----------------
@@ -56,6 +59,9 @@ The following main categories are defined:
 * LOGC_DT - Related to device tree control
 * LOGC_EFI - Related to EFI implementation
 
+To continue a log message in a separate call of function log() use
+
+* LOGC_CONT - Use same category as in previous call
 
 Enabling logging
 ----------------