[bootstrap] add support for Ubuntu on RaspberryPi (#6647)
[bootstrap] add support for Ubuntu on RaspberryPi (#6647)

Summary of changes:
- Enable building OpenThread RCP image on the Raspberry Pi (aarch64)
  directly (using Ubuntu)
- Fix problem with not having access to the /opt folder as a user
abtink/openthread,jwhui/openthread,abtink/openthread,openthread/openthread,jwhui/openthread,srickardti/openthread,jwhui/openthread,srickardti/openthread,jwhui/openthread,abtink/openthread,srickardti/openthread,openthread/openthread,abtink/openthread,openthread/openthread,srickardti/openthread,openthread/openthread