azure: ensure early boot messages are directed to the serial console
    
    In the event that there's an issue with early boot, setting
    earlyprintk=ttyS0 ensures that the messages are routed to the serial
    console so it can be debugged.
    
    This particular option is documented as required in the Azure VM
    certification FAQ[0].
    
    [0] https://learn.microsoft.com/en-us/partner-center/marketplace/azure-vm-certification-faq#linux-test-cases