git issue in update_external_sources.sh script

[shihhsinliicloud]

Reporting some issues with update_external_sources.sh script:

1. update_external_sources.sh script is not in the root of the repository, but in the decoder sample, either the build instructions need be updated or move the script to the root.
2. update_external_sources.sh script does a "git fetch --all" which breaks if you have a .gitconfig which references other repos. Changing to "git fetch origin" solve the problem.