The gateware version 5 does not have anymore the selection
between internal or external trigger. Instead, there is the
possibility to have more that one trigger source enabled.

Internals:
- the acquisition metadata now provides a whiterabbit timestamp
  and information about the trigger source that started the acquisition

Registers change:
- one register to enable/disable all trigger sources
- one register to set the polarity on all the triggers
- threshould/hysteresis for each channel
- delay on the following triggers: ext, channel[1; 4]

Sysfs changes
- add attributes to configure threshould
- trigger "enable" will restore the last known enable status