tools: Only issue a single Ctrl-C when entering raw REPL.

A long time ago when there was only the `stm` port, Ctrl-C would trigger a
preemptive NLR jump to break out of running code.  Then in commit
124df6f8d0 a more general approach to
asynchronous `KeyboardInterrupt` exceptions was implemented, and `stmhal`
supported both approaches, with the general (soft) interrupt taking
priority.

Then in commit bc1488a05f `pyboard.py` was
updated with a corresponding change to make it issue a double Ctrl-C to
break out of any existing code when entering the raw REPL (two Ctrl-C
characters were sent in order to more reliably trigger the preemptive NLR
jump).

No other port has preemptive NLR jumps and so a double Ctrl-C doesn't
really behave any differently to a single Ctrl-C: with USB CDC the double
Ctrl-C would most likely be in the same USB packet and so processed in the
same low-level USB callback, so it's just setting the keyboard interrupt
flag twice in a row.  The VM/runtime then just sees one keyboard interrupt
and acts as though only one Ctrl-C was sent.

This commit changes the double Ctrl-C to a single Ctrl-C in `pyboard.py`
and `mpremote`.  That keeps things as simple as they need to be.

Signed-off-by: Damien George <damien@micropython.org>

tools/mpremote/mpremote/transport_serial.py

@@ -121,7 +121,7 @@ class SerialTransport(Transport):
         return data
 
     def enter_raw_repl(self, soft_reset=True):
-        self.serial.write(b"\r\x03\x03")  # ctrl-C twice: interrupt any running program
+        self.serial.write(b"\r\x03")  # ctrl-C: interrupt any running program
 
         # flush input (without relying on serial.flushInput())
         n = self.serial.inWaiting()

tools/pyboard.py

@@ -348,7 +348,7 @@ class Pyboard:
         return data
 
     def enter_raw_repl(self, soft_reset=True):
-        self.serial.write(b"\r\x03\x03")  # ctrl-C twice: interrupt any running program
+        self.serial.write(b"\r\x03")  # ctrl-C: interrupt any running program
 
         # flush input (without relying on serial.flushInput())
         n = self.serial.inWaiting()