logd: allow opting into simple or serialized log buffers

Test: set properties and see logd use the appropriate buffer.
Change-Id: I2dcce7d9661a43adfafb69427d9271ee7ee29f43
diff --git a/logd/README.property b/logd/README.property
index ab9c4d4..1d7d990 100644
--- a/logd/README.property
+++ b/logd/README.property
@@ -52,6 +52,9 @@
 log.tag.<tag>             string persist The <tag> specific logging level.
 persist.log.tag.<tag>      string build  default for log.tag.<tag>
 
+logd.buffer_type           string (empty) Set the log buffer type.  Current choices are 'simple',
+                                          'chatty', or 'serialized'.  Defaults to 'chatty' if empty.
+
 NB:
 - auto - managed by /init
 - bool+ - "true", "false" and comma separated list of "eng" (forced false if