I think I finally solved this. Basically not all files from /lib/firmware/ are copied to the initrd image, which means that there's some mechanism, which decides what to add (and I have no idea what that is). Looking for info, I came across some suggestions that only the drivers compiled as kernel modules are subjected to this behavior. So basically when you've built a driver into the kernel, you have to take care of everything manually. So in my case, I have every single module built into my kernel, so there's two ways to handle this. I can use the CONFIG_EXTRA_FIRMWARE_DIR= and CONFIG_EXTRA_FIRMWARE= kernel options, or I can add a hook to initramfs config (under /etc/initramfs-tools/hooks/), which would look like the one following:
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#!/bin/sh -e
# Copy missing firmware files
PREREQ=""
prereqs () { echo "${PREREQ}"; }
case "${1}" in prereqs) prereqs; exit 0 ;; esac ;
. /usr/share/initramfs-tools/hook-functions
echo -n "Copying missing firmware files... "
[ ! -d "${DESTDIR}/lib/firmware/" ] && mkdir -p ${DESTDIR}/lib/firmware/
[ ! -d "${DESTDIR}/lib/firmware/brcm/" ] && mkdir -p
${DESTDIR}/lib/firmware/brcm/
cp /lib/firmware/iwlwifi-6000g2a-6.ucode ${DESTDIR}/lib/firmware/
cp /lib/firmware/brcm/BCM20702A1-0a5c-21e6.hcd ${DESTDIR}/lib/firmware/brcm/
echo "done."
exit 0
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And then the firmware files would finally get into the initrd image:
# lsinitramfs /boot/initrd.img-5.5.8-amd64 | grep -i firm
usr/lib/firmware
usr/lib/firmware/brcm
usr/lib/firmware/brcm/BCM20702A1-0a5c-21e6.hcd
usr/lib/firmware/iwlwifi-6000g2a-6.ucode
usr/lib/udev/rules.d/50-firmware.rules
which I think solves the issue for good. :]
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