hammer.py: support for Arch Linux

Optional of #2112 (closed)

Speaking of unsupported systems, I would like to enable users to run hammer commands in Arch Linux. This is a first step in that direction. It allows for ./hammer.py ensure-hammer-deps and ./hammer.py prepare-system. The second one only installs libyang and sysrepo for now.

This requires adding "ID_LIKE" as fallback to "ID" and adding "BUILD_ID" as fallback to "VERSION_ID", in /etc/os-release. "ID" and "VERSION_ID" are still prioritized as before.