Boot partition too small when installing realtime kernel

[tanius]

There is an issue with the instructions to install the PREEMPT_RT Linux kernel on a Raspberry Pi 3:

The /boot partition is by default only 41 MiB in size, which is not enough when having this kernel installed in parallel.

Whenever update-initramfs needs to run, it will complain that not enough disk space is available on /boot (to create a backup of the current initramfs image). This happens even without updating the kernel via apt, for example on updates to the initramfs-tools package. So it has to be resolved.

As a workaround, it is possible to free up enough space by removing the files /boot/kernel.img and /boot/kernel7.img. These are the default Raspberry Pi 1 / 2 resp. Raspberry Pi 3 kernels, not used as we configured kernel=vmlinuz-4.14.14-rt10-v7 in /boot/config.txt. The files belong to package raspberrypi-kernel, which we (probably) can't uninstall as it also contains several firmware images etc. in /boot.

As a proper solution, the /boot partition should simply be made larger right from the start.