This includes setting up everything for the mainline Raspberry Pi 4
image.
In fact, the only difference left in the Raspberry Pi 4-specific image
is the kernel from the vendor.
This should have been done initially, as otherwise it gets awfully
awkward to boot into new generations by default.
This system-specific image wasn't expected to be long-lived, thus why it
didn't end up being polished much.
Reality shows us we may be stuck with it for a bit longer, so let's make
it easier to use for new users.
