log: syslog driver

Provide a log driver that broadcasts RFC 3164 messages to syslog servers.
rsyslog is one implementation of such a server.

The messages are sent to the local broadcast address 255.255.255.255 on
port 514.

The environment variable log_hostname can be used to provide the HOSTNAME
field for the messages. The optional TIMESTAMP field of RFC 3164 is not
provided.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/README.log b/doc/README.log
index 19856d4..1057981 100644
--- a/doc/README.log
+++ b/doc/README.log
@@ -147,7 +147,10 @@
 enabled or disabled independently:
 
    console - goes to stdout
+   syslog - broadcast RFC 3164 messages to syslog servers on UDP port 514
 
+The syslog driver sends the value of environmental variable 'log_hostname' as
+HOSTNAME if available.
 
 Log format
 ----------