Increase boot partition size for classic gadget

[jawn-smith]

The following math was done to come up with the number of 512:

In [1]: fudge_factor = 2

In [2]: num_copies = 1 # we should only need one copy of the kernel for pc-amd64 images

In [3]: kernel_size = 11

In [4]: initrd_size = 169

In [5]: other_boot_assets = 22 # /boot/grub and /boot/EFI

In [6]: boot_size = fudge_factor * (num_copies * ( kernel_size + initrd_size +other_boot_assets))

In [7]: boot_size
Out[7]: 404

Then rounding up from 404 to a nice power of 2 we get 512.

[xnox]

num_copies = 1 # we should only need one copy of the kernel for pc-amd64 images

I don't think is true, since snapd does unpack two kernel.efi's during upgrade to ubuntu-boot partition for a/b booting. Or has this been changed somewhere else for this mode?

[jawn-smith]

@xnox thanks for taking a look! Is a/b booting something that is relevant for classic images? This PR is for the classic branch, which has a system-boot partition instead of ubuntu-boot.

That being said, I've looked into how current amd64 preinstalled server images are set up. It seems only /boot/efi is mounted on a separate partition, which doesn't include the kernel and initrd. The kernel and initrd come from the cloudimg-rootfs partition. This is not how the ubuntu-image redesign was planned to work, so I need to tag up with @sil2100 to see if the expectations of ubuntu-image always placing a kernel and initrd in the system-boot partition need to be reconsidered.