release: Comply with Azure Marketplace requirements for aarch64

Azure requires the first 1 MB (2,048 sectors) of the OS disk to remain
empty for VM images:

https://learn.microsoft.com/partner-center/marketplace-offers/azure-vm-certification-faq#vm-images-must-have-1-mb-of-free-space

Also append the BOOTPARTSOFFSET suffix for aarch64 images, which only
has an ESP partition for booting.

Co-authored-by: Brad Davis <brd@FreeBSD.org>
Approved by: cperciva
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D53628