Improve speed when ISO is running from physical DVD drive?

With our new-style ISOs, we achieve much higher boot speed by not copying the whole filesystem to RAM before booting.

However, on physical DVD drives this may have the disadvantage that the DVD drive needs to do a lot of non-sequential reads ("seeking"), which they are notoriously bad at.

Hence, we could possibly improve the speed when the ISO is running from a physical DVD drive.

If the computer has at least 3 GB and we are booted from an actual DVD drive, then copy to RAM before booting, unless the user says otherwise. Something like that.

(But then we would be back to reroot. And it would complicate the whole logic because we now would have to care for two different boot paths, which might increase the maintenance and testing effort down the line. So the question is: is it really worth the effort)

cc @larshenrikoern @jsm222