Zephyr: Chain loaded application has possibility of not working and faulting

[nordicjm]

There is an issue with the default mcuboot configuration when chainloading other (zephyr generally but any application is impacted) applications which is that by default, mcuboot does not clear the MPU configuration. When configured for an nrf52840, this enables a MPU stack guard by default which it places part the way through ram at 0x20002540 for 64 bytes, this memory is then made read only by the MPU. When the application starts, if it touches this ram, the module will fault - it is not known if an application will touch memory at this address or not but regardless the MPU must be cleared because leaving read only regions leads to undefined behaviour.