uts: fakebop should not output unprintable data
When prom_debug and/or kbm_debug is set, the early boot will produce debug output of set and discovered properties. The current code does interpret all properties as strings, which is not always true. Outputting binary data directly to terminal will interfere with terminal and can render terminal into bad state - such as activating alternate character sets and so on. So we have to make sure we do not output non-printable data to the terminal.