Signed-off-by: Michael Brown <mcb30@ipxe.org>

Use a private ANSI escape sequence to convey the priority of an
internal syslog() message through to the syslog server.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

Provide an internal syslog() function (unrelated to the syslog
console) which can be used to create log messages with specified
priorities.

The build-time constant LOG_LEVEL can be used to select the minimum
required priority for log messages.  Any messages that do not have a
sufficient priority will be ignored (and will be optimised away at
compile-time).

The default LOG_LEVEL is LOG_NONE.

Signed-off-by: Michael Brown <mcb30@ipxe.org>