tools/pyboard.py: Don't accumulate output data if data_consumer used.

Prior to this patch, when a lot of data was output by a running script
pyboard.py would try to capture all of this output into the "data"
variable, which would gradually slow down pyboard.py to the point where it
would have large CPU and memory usage (on the host) and potentially lose
data.

This patch fixes this problem by not accumulating the data in the case that
the data is not needed, which is when "data_consumer" is used.

tools/pyboard.py

@@ -261,6 +261,9 @@ class Pyboard:
         self.serial.close()
 
     def read_until(self, min_num_bytes, ending, timeout=10, data_consumer=None):
+        # if data_consumer is used then data is not accumulated and the ending must be 1 byte long
+        assert data_consumer is None or len(ending) == 1
+
         data = self.serial.read(min_num_bytes)
         if data_consumer:
             data_consumer(data)
@@ -270,9 +273,11 @@ class Pyboard:
                 break
             elif self.serial.inWaiting() > 0:
                 new_data = self.serial.read(1)
-                data = data + new_data
                 if data_consumer:
                     data_consumer(new_data)
+                    data = new_data
+                else:
+                    data = data + new_data
                 timeout_count = 0
             else:
                 timeout_count += 1