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.

With the new behaviour, log_debug() will always log if LOG_DEBUG is
enabled.

Move log_test_syslog_nodebug() into its own file since it cannot be made
to work where it is, with LOG_DEBUG defined.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/log.c b/common/log.c
index 9a5f100..ac34f1c 100644
--- a/common/log.c
+++ b/common/log.c
@@ -157,6 +157,9 @@
 {
 	struct log_filter *filt;
 
+	if (rec->force_debug)
+		return true;
+
 	/* If there are no filters, filter on the default log level */
 	if (list_empty(&ldev->filter_head)) {
 		if (rec->level > gd->default_log_level)
@@ -219,7 +222,8 @@
 	va_list args;
 
 	rec.cat = cat;
-	rec.level = level;
+	rec.level = level & LOGL_LEVEL_MASK;
+	rec.force_debug = level & LOGL_FORCE_DEBUG;
 	rec.file = file;
 	rec.line = line;
 	rec.func = func;