Extend boot partition for arm image types

The boot partition is used to stored the Linux kernel, and should be enough to store at least two versions of it to allow upgrades.

By default Apertis images allocate 256 MB for this partition, but this is getting increasingly tight once filesystem overhead is taken into account, with arm64 kernels requiring in the region of 115MB to install in the boot partition. In some cases, due to other items being stored in the boot partition, it is over 50% utilised with a single kernel installed.

As we expect the stock kernel sizes to continue to grow, especially the arm64 ones in no small part due to a growing number of device trees being built, increase the size of the boot partition to 512MB.

Signed-off-by: Martyn Welch martyn.welch@collabora.com