Increase size of /boot to 512 MiB

Increase size of /boot to 512 MiB

Is XFS is being selected as file system, the minimum size requirement is
300 MiB. In order to keep it to a round number, this patch increases the
size of /boot to 512 MiB.

To keep all systems consistent, we will also do this on systems that are
being formatted using different file systems.

Fixes: #13077 - xfs cannot installed anymore because boot is to small
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>