sample_updater: create tools/gen_update_config.py

gen_update_config.py generates JSON config files
from given OTA image zip files.

README.md is updated.

Test: manually

Change-Id: Ifd09b49a73983a42752ee3842a566cecedb9cae0
Signed-off-by: Zhomart Mukhamejanov <zhomart@google.com>
diff --git a/updater_sample/.gitignore b/updater_sample/.gitignore
index 487263f..f846472 100644
--- a/updater_sample/.gitignore
+++ b/updater_sample/.gitignore
@@ -7,3 +7,4 @@
 .idea/
 gen/
 .vscode
+local.properties