Add the RW-only firmware programmer for servo v2

Created a new main programmer class which provides programmer for only
updating the RW portion of BIOS with servo V2.

It does nothing on EC, as EC software sync on the next boot will
automatically overwrite the EC RW portion, using the EC RW image
inside the BIOS RW image.

BUG=chromium:645311
TEST=TBD

Change-Id: Ife660ecb76732aef13a9d27ed911ee9e6e4d2c95
Reviewed-on: https://chromium-review.googlesource.com/383772
Commit-Ready: Wai-Hong Tam <waihong@google.com>
Tested-by: Wai-Hong Tam <waihong@google.com>
Reviewed-by: danny chan <dchan@chromium.org>
diff --git a/server/cros/servo/servo.py b/server/cros/servo/servo.py
index 2b3f565..4fd8712 100644
--- a/server/cros/servo/servo.py
+++ b/server/cros/servo/servo.py
@@ -713,6 +713,7 @@
         servo_version = self.get_servo_version()
         if servo_version.startswith('servo_v2'):
             self._programmer = firmware_programmer.ProgrammerV2(self)
+            self._programmer_rw = firmware_programmer.ProgrammerV2RwOnly(self)
         elif servo_version.startswith('servo_v3'):
             self._programmer = firmware_programmer.ProgrammerV3(self)
             self._programmer_rw = firmware_programmer.ProgrammerV3RwOnly(self)