Move to "software defined page size"

[d3zd3z]

Remove per-sector query of page size, and make all of mcuboot use a software defined page size. This should allow improvements with variable page-size blocks, as well as allowing swap-move to work on all device, and better support for external devices with differing page sizes.

• Single query for erase and write sizes, consistent across entire partition.
• Definable in DT (on Zephyr) to override real sizes.
• Testing