Updating talker transmit OK to prevent occasional start delay.

We now check if it's the first packet to be sent and don't calculate the
elapsed time.
Tabs to spaces
Adding back the AT board XN file
Simplifying master clock selection for 2 XMOS devices
Delay 1722.1 start until PTP port has changed roles
Correcting runtime exception when disabling talker or listener
Clarify audio slice requirement in doc
Typo in doc
Updating descriptor string to beta4
'Release: 5.2.0beta4'
Correcting typo in variable name
Improving 1722.1 controller clock selection

Previously didn't handle the case where the Talker or Listener was
disabled
Updating descriptor version string - beta5
'Release: 5.2.0beta5'
Fix for ADP entity available messages on link down/up

State wasn't being correctly reset
Basic propagation now working.
Dest addr now stored in correct byte order
Merge branch 'master' into dc

Conflicts:
	module_avb/src/avb.c