[Build] Use ccache if it exists

This explicitly uses ccache and doesn't rely on it being in the FPP
user's path.  We may change this in the future to use the path but
for now this method is easier to revert if there are any issues.
[Startup] Log output of startup script

This adds a 'teeOutput' function to tee the output instead of just
logging to the log file as the 'logOutput' function does.
[Startup] Networking tweaks

- Mark eth0 down when it does not have link
- Allow setting 'ENABLED=0' in interface config file to disable an
  interface.  There is no UI for this, it is mainly for testing, but
  adding a UI could be done later if desired.
- Don't bring up the fallback IP on eth0 if there is no link
- Don't bring up the fallback IP on eth0 if there is a static IP
  configured.