Weston: log to syslog instead of /tmp/weston.log (#762)

Placing such files in /tmp/ is an unnecessary attack vector, as
@pavelmachek showed. All udev rules, which mentioned /tmp/weston.log
do not contain any path anymore.

Logs can be read with:
logread -f /var/log/messages | grep weston

See also: <https://wiki.alpinelinux.org/wiki/Syslog>