[logs] Cleanup some logging.

We get a lot of logs with some things that need cleanup.

• We don't need to print the interface, we have that in configuration
  files.

• When we check variables in fppxml and fppjson, only print the "empty"
  variables when debug is enabled.  empty() returns true for 0 values
  that are actually populated so there are a lot of false positives
  being reported and confusing users.

• Parse string error only printed in debug now

• Cleanup zip creation:
    · Check permissions/presence of system files.  Without this, we can
      end up with empty zips.
    · Using a proc file directly as a file can create a bad zip, get the
      output from cat and add the file as a string.

• Remove the "error RTC" log
[rf24] Stub the non-pi build.