tools/pyboard.py: Import errno to fix undefined name in PyboardError.

This will keep line 96 from raising a NameError.

Signed-off-by: Christian Clauss <cclauss@me.com>

tools/pyboard.py

@@ -68,6 +68,7 @@ Or:
 """
 
 import ast
+import errno
 import os
 import struct
 import sys