/etc/os-release was introduced by systemd[1] and is now
more and more common; even on systems without systemd (e.g. lede).
In addition to detecting the OS based on specific attributes,
this file provides the ID marker to describe the OS.
This commit adds support for OS detection via /etc/os-release.
According to [2], it is already lowercase.
[1] http://0pointer.de/blog/projects/os-release
[2] https://www.freedesktop.org/software/systemd/man/os-release.html
