feat(serial): Add function to switch serial to synchronous

In severe error conditions, asynchronous prints will never work.  For
such cases we need a way to make prints happen synchronously again, the
same way it works during early boot.  Add a serial_return_to_synchronous()
function which unconditionally switches the serial driver code to
synchronous mode.

Only use this function in unrecoverable error conditions!

Signed-off-by: Rahix <rahix@rahix.de>