raspberrypi2-kernel4-devel should be marked installonly

[lightdot]

Basically, as the title says. A manual solution is to add a installonlypkgs=raspberrypi2-kernel4-devel line to /etc/dnf/dnf.conf.

Without this, only a single raspberrypi2-kernel4-devel is kept as the newer kernel installs. Not having a corresponding raspberrypi2-kernel4-devel package for older raspberrypi2-kernel4 kernels causes issues e.g. when building custom kernel modules.