Adding an image viewer for quick image display

Listens to camera channel and displays images using imshow.

Works well with X forwarding on pi's (e.g., ssh -X 10.9.71.101)

Change-Id: I45431a7925aef9fcc5ee4275e367407ad264dea3
diff --git a/y2020/vision/BUILD b/y2020/vision/BUILD
index 5e65387..b95bca3 100644
--- a/y2020/vision/BUILD
+++ b/y2020/vision/BUILD
@@ -59,3 +59,24 @@
     srcs = ["vision.fbs"],
     visibility = ["//y2020:__subpackages__"],
 )
+
+cc_binary(
+    name = "viewer",
+    srcs = [
+        "viewer.cc",
+    ],
+    data = [
+        "//y2020:config.json",
+    ],
+    restricted_to = [
+        "//tools:k8",
+        "//tools:armhf-debian",
+    ],
+    visibility = ["//y2020:__subpackages__"],
+    deps = [
+        ":vision_fbs",
+        "//aos:init",
+        "//aos/events:shm_event_loop",
+        "//third_party:opencv",
+    ],
+)