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>