Enable support for building mcuboot for Mbed with direct-xip

This commit also introduces changes that allow users to build for other non-swap type update methods (overwrite only, swap using move, direct xip, or RAM loading). Changes include:

- Adding configuration options relating to XIP
- Updating the Mbed flash map backend to be compatible with XIP updates
- Add default secondary_bd in internal flash for XIP on Mbed OS.

Signed-off-by: George Beckstein <becksteing@embeddedplanet.com>