log: Add a flag to enable log drivers

At present there is no way to disable a log driver. But the syslog driver
causes (attempted) network traffic in sandbox every time a log message
is printed, which is often.

Add a flag to enable a log driver. Adjust struct log_device to use a short
for next_filter_num so that no more memory is used for devices. Also fix
a missing line in the struct log_driver comment while here.

To maintain compatibility, enable it for all drivers for now.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/log.c b/common/log.c
index ac34f1c..d6dfabb 100644
--- a/common/log.c
+++ b/common/log.c
@@ -207,7 +207,8 @@
 	/* Emit message */
 	processing_msg = 1;
 	list_for_each_entry(ldev, &gd->log_head, sibling_node) {
-		if (log_passes_filters(ldev, rec))
+		if ((ldev->flags & LOGDF_ENABLE) &&
+		    log_passes_filters(ldev, rec))
 			ldev->drv->emit(ldev, rec);
 	}
 	processing_msg = 0;
@@ -329,6 +330,7 @@
 		}
 		INIT_LIST_HEAD(&ldev->filter_head);
 		ldev->drv = drv;
+		ldev->flags = drv->flags;
 		list_add_tail(&ldev->sibling_node,
 			      (struct list_head *)&gd->log_head);
 		drv++;