bootutil: Add manifest-based loader for Direct XIP

[tomchy]

Add a loader variant that is capable of booting images in Direct XIP mode, based on a simple manifest.

• The manifest image, configured through MCUBOOT_MANIFEST_IMAGE_INDEX (slot pair) is the main/lead image - it describes firmware bundle for the device.
• The firmware bundle description is provided in the protected TLV of the manifest image. Images relations are expressed as their hashes. Hash of the manifest image is not in the TLV as it covers it as well.

For more details, please take a look at updates inside the design.md file.