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So that the user can explicitly deactivate UART(0) if needed.  See
issue #4314.

This introduces some risk to "brick" the device, if the user disables the
REPL without providing an alternative REPL (eg WebREPL), or any way to
reenable it.  In such a case the device needs to be erased and
reprogrammed.  This seems unavoidable, given the desire to have the option
to use the UART for something other than the REPL.