Wasm: Onboarding always appears for VRM read-only systems

[blammit]

Currently, the onboarding screen will always show on start-up when run via wasm on a VRM read-only system. This is because the system is unable to write to /Settings/Gui2/OnBoarding to clear the onboarding flag.

Need a straightforward way to determine whether the system is read-only.

One possible solution is to set a cookie in this case, instead of writing a value via MQTT.

[mpvader]

And another one is to get a parameter from VRM telling the wasm that its read only