Add userconf support to Raspbian

[oliverblaha]

Newer versions of Raspberry OS enforce interactive user setup, breaking headless setup. It's possible to opt out by having a userconf or userconf.txt file present (similar to ssh/ssh.txt for enabling the SSH server). This change adds support for copying this file during setup.

[oliverblaha]

Reference: https://github.com/raspberrypi/documentation/blob/develop/documentation/asciidoc/computers/configuration/headless.adoc

[oliverblaha]

... just wondering: is the Raspbian folder actually the correct one to override recent Raspberry OS image partition setups, or would this require adding an override?

[procount]

No it's not the correct folder, but I'll sort it. Thanks for the PR.