main/devicepkg-dev: add hardware vendor & model into machine-info

Soon openrc-settingsd (hostnamed) will uses these properties so the information can be exposed to apps like GNOME Settings.

Let's also move the shellcheck comment to the outer scope, otherwise we'd need four of these comments now, and let's not use ${} but just $.

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Results in a /etc/machine-info file like this:

PRETTY_HOSTNAME="Fairphone 4"
CHASSIS="handset"
HARDWARE_VENDOR="Fairphone"
HARDWARE_MODEL="Fairphone 4"

See also https://gitlab.com/postmarketOS/openrc-settingsd/-/merge_requests/4 , can be merged independently.