Build and use ota_from_target_files as a module.

Bug: 63866463
Test: TreeHugger
Test: m -j otapackage
Test: `m -j otatools-package`. Check bin/ota_from_target_files is
      available. Build an OTA package with standalone otatools.
Change-Id: I533dee1c6997d233601e7f4eae637ff5743ed637
diff --git a/tools/releasetools/Android.bp b/tools/releasetools/Android.bp
index 40cdce8..34e75c0 100644
--- a/tools/releasetools/Android.bp
+++ b/tools/releasetools/Android.bp
@@ -119,6 +119,22 @@
     ],
 }
 
+python_binary_host {
+    name: "ota_from_target_files",
+    defaults: ["releasetools_binary_defaults"],
+    srcs: [
+        "edify_generator.py",
+        "ota_from_target_files.py",
+    ],
+    libs: [
+        "releasetools_common",
+        "releasetools_verity_utils",
+    ],
+    required: [
+        "brillo_update_payload",
+    ],
+}
+
 python_defaults {
     name: "releasetools_test_defaults",
     srcs: [