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