log: Handle line continuation

When multiple log() calls are used which don't end in newline, the
log prefix is prepended multiple times in the same line. This makes the
output look strange.

Fix this by detecting when the previous log record did not end in newline.
In that case, setting a flag.

Drop the unused BUFFSIZE in the test while we are here.

As an example implementation, update log_console to check the flag and
produce the expected output.

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

doc/develop/logging.rst

@@ -96,6 +96,22 @@ Also debug() and error() will generate log records  - these use LOG_CATEGORY
 as the category, so you should #define this right at the top of the source
 file to ensure the category is correct.
 
+Generally each log format_string ends with a newline. If it does not, then the
+next log statement will have the LOGRECF_CONT flag set. This can be used to
+continue the statement on the same line as the previous one without emitting
+new header information (such as category/level). This behaviour is implemented
+with log_console. Here is an example that prints a list all on one line with
+the tags at the start:
+
+.. code-block:: c
+
+   log_debug("Here is a list:");
+   for (i = 0; i < count; i++)
+      log_debug(" item %d", i);
+   log_debug("\n");
+
+Also see the special category LOGL_CONT and level LOGC_CONT.
+
 You can also define CONFIG_LOG_ERROR_RETURN to enable the log_ret() macro. This
 can be used whenever your function returns an error value: