[console] Add "log message" console usage and an internal syslog() call

[console] Add "log message" console usage and an internal syslog() call

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>