Region copy buffer size not actually configurable

[kail]

This issue is both a bug fix and feature request. I have opened a PR with a proposed fix to address this, and open to feedback there. I did not see any duplicates of this, and happy to close/link this issue in favor of an existing one.

Issue:

This PR introduced a change which appeared to be intended to allow the buffer size to be configurable.

That change, however, is frivolous because MCUBOOT_BOOT_MAX_ALIGN is guaranteed to be between 8 and 32