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>