sys-boot/raspberrypi-firmware: don't install device tree blobs

sys-boot/raspberrypi-firmware: don't install device tree blobs

Upstream git repo provides binaries of two natures in a single
repository:

* boot firmware files;
* prebuilt kernel files.

boot/*.dtb, boot/overlays/ belong to kernel, not boot firmware.

But for historical reasons, these files were installed by
sys-boot/raspberrypi-firmware package rather than
sys-kernel/raspberrypi-image, which would be more correct.

The problem with this is that users of kernels different than
sys-kernel/raspberrypi-image need to install different files into these
locations. This means such people have to avoid using
sys-boot/raspberrypi-firmware package completely.

A blocker dependency on old sys-kernel/raspberrypi-image versions is
added to protect from a situation when just raspberrypi-firmware is
being upgraded, and the machine ends up not having device tree files.

Upgrading to a new version may cause a disruption if the user doesn't
use sys-kernel/raspberrypi-image but still relies on device tree files
installed by raspberrypi-firmware package. This will be dealt with by a
news item. A special USE flag for smooth transition of such users is
considered an overkill, on presumption that this package is not
massively popular.

Package-Manager: Portage-2.3.66, Repoman-2.3.16
Signed-off-by: Andrey Utkin <andrey_utkin@gentoo.org>