Minor update to SD image creation script.

Master image has a text file in /home/pi so it's obvious when
you're working on the master if you have 4 Pi's on your desk
and are logged into all 4 remotely.  Clean that file up when
we package the SD image.
Update doc tracking changes to stock OS image.
Minor tweaks to rasclock doc and Makefile.

There is more to add to this that will make compiling this module
even easier, but I still need to write it up.
Fix E1.31 and FPD output in some situations.

On a fresh image, with no other (USB/SPI) channel outputs defined,
E1.31 and FPD output would not work.  This commit fixes that issue
reported on the forum.

Includes new fppd binary for stage branch.
Default DNS setting to DHCP since that's the default with no config.
Startup script enhancements.

- Add a work-around for audio issues in older Raspberry firmware.
  https://github.com/raspberrypi/linux/issues/570

- Move a few things around to give DHCP a little more time to come
  up before we log/announce the IP addresses.  This way we're doing
  something rather than just waiting in WaitForDHCPIPs for the IP(s)
  to come up.

- Disable wifi interface by default on stock SD image so that
  WaitForDHCPIPs doesn't wait 60 seconds for it to get an IP address.
  When users configure wifi via the Network Config screen, we'll
  detect that and wait for an IP to come up on the wireless interface.

- Execute flite in the background so that we can proceed with bootup
  while it announces our IP addresses.  Shaves another few seconds off
  our boot time.
Add the silence.mp3 file referenced by the last commit.
Don't bring up the fallback static IP if we have a good one already.

If we have a default gateway on eth0 and can ping it, then don't
bother bringing up our fallback static IP address since it can
complicate other things and even prevent some services such as Avahi
from functioning properly on a network with multiple FPP instances.