Bug#985956: Merge request submittted to initramfs-tools

[Pete Batard <pete@akeo.ie>]

Note that this is a rather critical regression (since it used to work 
fine with previous bullseye ISOs), that is currently preventing 
installation of Debian on the Raspberry Pi 4, i.e. by far the most 
widespread ARM64 platform out there.

Quite a few people are negatively affected by this bug, and one can 
expect a lot more to be if Debian 11 goes to release without the 
inclusion of mdio-bcm-unimac.ko in the netinst ARM64 ISOs.

I have created an initramfs-tools merge request to attempt to fix the 
problem, which can be found at:
https://salsa.debian.org/kernel-team/initramfs-tools/-/merge_requests/46

However, since I am not entirely familiar with how netinst ISO kernel 
modules are generated and picked, I can't vouch that the proposed fix, 
which consists of referencing the mdio/ subdirectory for inclusion, is 
the proper way to go...