rsyslog: Add config files and use update-alternatives for rsyslog

- Add a logrotate config file for rsyslog.
- Change rsyslog.conf to be compatible with sysklogd.conf.
- Use update-alternatives since we have other syslog systems:
  sysklogd, busybox-syslog, etc.

Signed-off-by: Jackie Huang <jackie.huang@windriver.com>