Add tests for imgdiff.

Factor out libimgdiff static library for testing purpose.

This CL adds the imgdiff tests on host and on target both (similar to
libimgpatch). In practice, we only need imgdiff binary on host, and
libimgpatch on target. But they should build and pass tests on both
platforms.

Test: recovery_host_test passes; recovery_component_test passes.
Change-Id: I0eafb7faf727cdf70066310e845af6ee245d4f60
diff --git a/applypatch/imgdiff_main.cpp b/applypatch/imgdiff_main.cpp
new file mode 100644
index 0000000..7d5bdf9
--- /dev/null
+++ b/applypatch/imgdiff_main.cpp
@@ -0,0 +1,21 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "applypatch/imgdiff.h"
+
+int main(int argc, char** argv) {
+  return imgdiff(argc, const_cast<const char**>(argv));
+}