[SYSLOG] Update and extend the SYSLOG to support buffering and logging to files.

[jameswalmsley]

Currently the SYSLOG api (BT_kPrint() and BT_kDebug()) simply write immediately to the kernel console.

This should be extended to provide a fully featured syslog api, complete with syscall allowing applications to log to the system log.

[jameswalmsley]

This is dependent on #12