tools/pyboard.py: Refactor so target is not reset between scripts/cmd.

Previous to this patch pyboard.py would open a new serial connection to
the target for each script that was run, and for any command that was run.
Apart from being inefficient, this meant that the board was soft-reset
between scripts/commands, which precludes scripts from accessing variables
set in a previous one.

This patch changes the behaviour of pyboard.py so that the connection to
the target is created only once, and it's not reset between scripts or any
command that is sent with the -c option.

tools/pyboard.py

@@ -285,43 +285,66 @@ def main():
     cmd_parser.add_argument('files', nargs='*', help='input files')
     args = cmd_parser.parse_args()
 
-    def execbuffer(buf):
+    # open the connection to the pyboard
     try:
         pyb = Pyboard(args.device, args.baudrate, args.user, args.password, args.wait)
+    except PyboardError as er:
+        print(er)
+        sys.exit(1)
+
+    # run any command or file(s)
+    if args.command is not None or len(args.files):
+        # we must enter raw-REPL mode to execute commands
+        # this will do a soft-reset of the board
+        try:
             pyb.enter_raw_repl()
+        except PyboardError as er:
+            print(er)
+            sys.exit(1)
+
+        def execbuffer(buf):
+            try:
                 ret, ret_err = pyb.exec_raw(buf, timeout=None, data_consumer=stdout_write_bytes)
-            pyb.exit_raw_repl()
-            pyb.close()
             except PyboardError as er:
                 print(er)
                 sys.exit(1)
             except KeyboardInterrupt:
                 sys.exit(1)
             if ret_err:
+                pyb.exit_raw_repl()
+                pyb.close()
                 stdout_write_bytes(ret_err)
                 sys.exit(1)
 
+        # run the command, if given
         if args.command is not None:
             execbuffer(args.command.encode('utf-8'))
 
+        # run any files
         for filename in args.files:
             with open(filename, 'rb') as f:
                 pyfile = f.read()
                 execbuffer(pyfile)
 
+        # exiting raw-REPL just drops to friendly-REPL mode
+        pyb.exit_raw_repl()
+
+    # if asked explicitly, or no files given, then follow the output
     if args.follow or (args.command is None and len(args.files) == 0):
         try:
-            pyb = Pyboard(args.device, args.baudrate, args.user, args.password, args.wait)
             ret, ret_err = pyb.follow(timeout=None, data_consumer=stdout_write_bytes)
-            pyb.close()
         except PyboardError as er:
             print(er)
             sys.exit(1)
         except KeyboardInterrupt:
             sys.exit(1)
         if ret_err:
+            pyb.close()
             stdout_write_bytes(ret_err)
             sys.exit(1)
 
+    # close the connection to the pyboard
+    pyb.close()
+
 if __name__ == "__main__":
     main()