Update the `wpanreatin` script to use/retain Router Id
Update the `wpanreatin` script to use/retain Router Id

This commit changes the `wpanretain.sh` script to use the "Router
Id".

d-bahr/wpantund,aeliot/wpantund,openthread/wpantund,aeliot/wpantund,openthread/wpantund,abtink/wpantund,d-bahr/wpantund,aeliot/wpantund,abtink/wpantund,openthread/wpantund,abtink/wpantund,d-bahr/wpantund,aeliot/wpantund,abtink/wpantund,d-bahr/wpantund,openthread/wpantund