support different boot partition sizes in aosp_arm64

The different boot images in aosp_arm64 have different partition size
requirements. Add support for defining a partition size associated with
each boot image variant.

This support is limited to aosp_arm64 currently, which is built with
recovery-as-boot.

A previous version of this change did not update the boot-debug image
AVB logic with the required macro for multiple boot partition size
support. This is now required because the aosp-arm64 boot.img is
configured as a chained AVB partition.

Bug: 156036850
Bug: 155049180
Change-Id: I66b57de91042bfd56ba54a3659843d8cf7873955