boot: Introduce direct execute-in-place (XIP) mode

This patch introduces the direct execute-in-place (XIP) mode in addition
to the other upgrade modes (swap strategies, overwrite-only). When
direct-XIP is enabled with the MCUBOOT_DIRECT_XIP option, mcuboot
selects the newest valid image based on the image version numbers from
the image header, thereafter the selected image runs directly from its
flash partition (slot) instead of moving it. Therefore the images must
be linked to be executed from the given image slot. It means that in
direct-XIP mode either of the primary and the secondary slots can hold
the active image.

This patch is based on the NO_SWAP upgrade strategy which was first
introduced in the Trusted Firmware-M project.
Source TF-M version: TF-Mv1.0.

Change-Id: If584cf01ae5aa7208845f6a6fa206f0595e0e61e
Signed-off-by: David Vincze <david.vincze@linaro.org>