boot: Add flash area ID/device ID retrieval hooks

For users that customise (for instance, via boot_go() hook) from where
images are being loaded from, having those hardcoded to
`slot_partitionX` (when using Zephyr) can be problematic. New hooks,
`flash_area_id_from_multi_image_slot_hook` and
`flash_area_get_device_id_hook` allow some customisation around these
processes.

Support for these hooks is shielded by a different configuration than
other hooks - MCUBOOT_FLASH_AREA_HOOKS, so that it can be enabled
independently from other available hooks.

Support for this new hook was added to the Zephyr port.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>