STK500v2: Retries for reading from serial

This would normally not fly at all but we give it a pass here since we
only use that module for auto detection anyhow (and probably not for
much longer since that approach is simply too error prone).

Adapted from

  https://github.com/foosel/OctoPrint/issues/2138#issuecomment-334117910

by @dtynan.

Solves #2138 until I come up with something better

src/octoprint/util/avr_isp/stk500v2.py

@@ -111,8 +111,11 @@ class Stk500v2(ispBase.IspBase):
 		state = 'Start'
 		checksum = 0
 		while True:
-			s = self.serial.read()
-			if len(s) < 1:
+			for _ in range(0, 5):
+				s = self.serial.read()
+				if len(s) > 0:
+					break
+			else:
 				raise ispBase.IspError("Timeout")
 			b = struct.unpack(">B", s)[0]
 			checksum ^= b