Add firmware for built-in drivers

There's an old bug report from 2017 (#857054) complaining that firmware for
built-in drivers is not automatically added to initramfs; and back then,
the kernel did not collect information about such dependencies in a place
where initramfs-tools could access it (#857055).

However, since Linux 5.2 (released in mid-2019), the kernel actually
provides this information via modules.builtin.modinfo; you can see at e.g.
<https://packages.debian.org/bullseye/amd64/linux-image-5.7.0-3-amd64/filelist>
that the buster kernel package ships this file. (There aren't actually any
firmware dependencies listed in there though; I guess the upstream Debian
kernel config just has no built-in code with firmware dependencies?)

Wire it up, so that people with custom kernel configs don't have to waste
time figuring out that their devices don't work properly because the
firmware wasn't available at boot.

Closes: #857054