Add static uid to aziotd + aziot-edged users

[emilm]

Not really a bug but it would make adding config.toml to each service easier because of "[[principal]]".

my own certd.toml:

[[principal]]
uid = 987
certs = [...]

aziot-edged_ ... .bb : USERADD_PARAM_${PN} = "-r -g iotedge -c -u 987 'iotedge user'

You already did this on edgeagentuser and edgehubuser

Also maybe add the ability to have everything as "root" ? What are the dangers of doing this on an unattended embedded system?