Add logging #57
Description
pslab-system should provide a logging mechanism.
Log sink should be configurable, and could be e.g. a UART bus or (in the future when we add SD-card support) a file on an SD-card. Logging could also be disabled entirely, in which log calls are no-ops.
Log sink configuration can be done at compile time for now. In the future, we can add a configuration parameter in flash.