doc(build): explanation on git submodules and refactor of sync script

• Explanation on the git submodule commands
• Refactor of scripts

docs/BUILD_IMAGES.md

@@ -154,7 +154,7 @@ We are supporting the following releases:
 * Ubuntu `20.04` or `focal`
 * Ubuntu `22.04` or `jammy`
 
-When building, you can specify which base image to work on (by default it will be `ubuntu:jammy`).
+When building, you can specify which base image to work on (by default it will be `ubuntu:focal`).
 
 You just add the `--build-arg BASE_IMAGE=ubuntu:xxxx` option.