tools/pyboard: Run exec: command as a string.

The Python documentation recommends to pass the command as a string when
using Popen(..., shell=True).  This is because "sh -c <string>" is used to
execute the command and additional arguments after the command string are
passed to the shell itself (not the executing command).

https://docs.python.org/3.5/library/subprocess.html#subprocess.Popen

tools/pyboard.py

@@ -152,7 +152,7 @@ class ProcessToSerial:
 
     def __init__(self, cmd):
         import subprocess
-        self.subp = subprocess.Popen(cmd.split(), bufsize=0, shell=True, preexec_fn=os.setsid,
+        self.subp = subprocess.Popen(cmd, bufsize=0, shell=True, preexec_fn=os.setsid,
             stdin=subprocess.PIPE, stdout=subprocess.PIPE)
 
         # Initially was implemented with selectors, but that adds Python3