log: Add filter flag to deny on match

Without this flag, log filters can only explicitly accept messages.
Allowing denial makes it easier to filter certain subsystems. Unlike
allow-ing filters, deny-ing filters are added to the beginning of the
filter list. This should do the Right Thing most of the time, but it's
less-universal than allowing filters to be inserted anywhere. If this
becomes a problem, then perhaps log_filter_add* should take a filter number
to insert before/after.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/common/log.c b/common/log.c
index b7b144f..c1b6b61 100644
--- a/common/log.c
+++ b/common/log.c
@@ -170,7 +170,11 @@
 		if (filt->file_list &&
 		    !log_has_file(filt->file_list, rec->file))
 			continue;
-		return true;
+
+		if (filt->flags & LOGFF_DENY)
+			return false;
+		else
+			return true;
 	}
 
 	return false;
@@ -284,7 +288,11 @@
 		}
 	}
 	filt->filter_num = ldev->next_filter_num++;
-	list_add_tail(&filt->sibling_node, &ldev->filter_head);
+	/* Add deny filters to the beginning of the list */
+	if (flags & LOGFF_DENY)
+		list_add(&filt->sibling_node, &ldev->filter_head);
+	else
+		list_add_tail(&filt->sibling_node, &ldev->filter_head);
 
 	return filt->filter_num;