* add missing libelf-dev dependency (to build x86-64)
* remove subversion dependency
* add rsync dependency
* add ecdsautils to docs

This has become possible with OpenWrt 21.02 and while the Dockerfile
already received that updated, we forgot to update the CI and the
documentation.

apt does not have a stable CLI interface. Don't use it in scripts.

- Rely on shebang instead of setting shell in workflow
- Run whole install-dependencies.sh in sudo
- Use /bin/sh instead of bash
- set -e