scripts/convert-srcuri: Backport SRC_URI conversion script from master branch

This script handles two emerging issues:

1. There is uncertainty about the default branch name in git going forward.
To try and cover the different possible outcomes, add branch names to all
git:// and gitsm:// SRC_URI entries.

2. Github are dropping support for git:// protocol fetching, so remap github
 urls as needed. For more details see:

https://github.blog/2021-09-01-improving-git-protocol-security-github/



Signed-off-by: Steve Sakoman <steve@sakoman.com>