Allow configuring endpoint 1 selection behavior

[puddly]

Supersedes #175

@prairiesnpr I gave this some thought and I think tying the current endpoint selection logic to whether or not extra endpoints exist will modify the default behavior in cases where this will be necessary (i.e. testing a new, weird device). Instead, a new config option is used:

zha:
  zigpy_config:
    znp_config:
      prefer_endpoint_1: false