Allow disabling copying of udev rules

[zaninime]

Currently, the builder builds the whole system.build.etc, which is a rather enormous closure in some configurations. Also, it requires the build of all packages declared in systemPackages for the build architecture, which is in many cases unneeded, and can fail if the packages are platform-specific.

[Lassulus]

We usually need the udev rules to create the /dev/disk/by-partlabel symlinks. So without them disko falls apart a bit, so I'm curious what your usecase is and why you don't need them :)

maybe a better approach would be, to just copy the udev rules we need instead of all of them?

[zaninime]

My usecase was building an image for Raspberry Pi, and with this (and the other PRs I opened) I was successful. :)

I do see in my fstab:

/dev/disk/by-partlabel/system /system ext4 x-initrd.mount,noatime 0 2

where system is one of the partitions managed by disko, so this has to be working also without udev rules. I omitted the other partitions for brevity.

In my case particularly there's some software that is not available in caches and I do not wish to rebuild it in the cross configuration just for the sake of building the image. It has no impact on udev rules. One example: https://github.com/raspberrypi/rpicam-apps depends on a custom ffmpeg, qt, ... and eventually even fails to compile in the cross config.