[RFC] Support for choosing image to boot in runtime

[edersondisouza]

MCUboot assumes that the images being booted live on a flash (or will be loaded to it, in case of serial support) or something that pretends to be a flash. It will usually check version of available images on flash partition slots, and choose the newer one to perform an update, with support to fallback to a previous working image in case the new one fails.

Another use cause for MCUboot, however, is to be able to load an image from an arbitrary source, in fact, not caring about the update facilities. In that case, the application will usually check some hardware straps to decide from which source to get the image to boot.

This RFC PR adds support for such scenarios: it uses single loader to load an image from a set of "sources". The single loader loops through available sources and the first one to succeed signature/validation boots. To access the images, weak functions flash_map_id_get_next() and flash_map_id_get_current() are used. Default implementation keeps current behaviour for single loader, i.e. just loads from FLASH_AREA_IMAGE_PRIMARY(0).

It is expected applications will reimplement these functions, allowing them to define a priority of different sources. As these different storage media may be ready only, MCUboot won't attempt to update them to record last source to succeed or so, it's application responsibility to define the correct priority of sources on every boot.

This PR also moves RAM loading code to its own file, so it's available for single loaders, as well as provide one example of such approach, using two slots on a FRDM-K64F.

While some of the source devices can not be flash devices at all (for instance, image is made available via I2C or eSPI bus), feedback on #2031 supports simply implementing the Flash API for them at Zephyr level, hence this new RFC PR.

Please review - comments and suggestions on how to better achieve this are highly appreciated =D

This is a follow on for #2031.