boot/bootutil: Remove direct accesses to flash_area::fa_xyz

[adri326]

I plan on working towards building a Rust API for bootutil, and I would like to pass an opaque struct flash_area to bootutil, so that this struct can be fully managed from the rust side, with only a few extern "C" functions that will need to be defined on the rust side before linking.

This PR removes the few direct accesses to flash_area::fa_id, flash_area::fa_off and flash_area::fa_size that are present in bootutil, and replaces them with the corresponding flash_area_get_XYZ functions.

Currently, however, one can only use an opaque struct flash_area if they enable the new flash sector API (MCUBOOT_USE_FLASH_AREA_GET_SECTORS), since otherwise loader.c will attempt to create an array of struct flash_area. I thus do not know how we could ensure that no additional mentions of flash_area::fa_XYZ will sneak back into the code.

One option would be to create a bootutil_shared.h file, which would contain the minimal expected subset of flash_map_backend.h, and progressively have files include it, until the flash_map_backend.h only needs to provide the struct flash_area for when MCUBOOT_USE_FLASH_AREA_GET_SECTORS isn't toggled on.

[de-nordic]

@adri326 Do not get used to flash_area in MCUboot: https://github.com/orgs/mcu-tools/projects/1?pane=issue&itemId=40095438

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