With the existing code problems can occur with remounting, the "if t -
t_last_activity > QUIET_TIMEOUT:" check can be triggered early before the
REPL string comes through, meaning that the remount doesn't happen.
On certain boards the "MPY: soft reboot" line comes through immediately
(getting the routine past initial timeout) but then there's a slightly
longer delay while the board restarts before it prints out the startup
header and the REPL prompt.
This commit adds some extra pattern monitoring during the timeout loop to
track the state if a soft restart is actually started.
79c05bd522