Necessary to fix "mpremote run" over hardware UART.
Bisect shows bug was introduced by d420b4e4, but looks like made more
complex by 01c046d2. Specifically: resetting and re-initialising the REPL
UART during soft reset clears the FIFO before it's done printing the "MPY:
soft reboot" line.
Fixed by adding a UART TX flush in the deinit path.
Signed-off-by: Angus Gratton <angus@redyak.com.au>
73feaaf931