Fix corruption of mcuboot-s1 and netcore partitions

upgradable mcuboot bootloader option, to validate and select which mcuboot slot to boot:
optional trailer at the end of of flash partitions for mcuboot slot0 and slot1

1. mcuboot erase flash sector for mcuboot_image_trailer
2. mcuboot writes new image to flash (e.g. "mcumgr image upload")
3. mcuboot writes mcuboot_image_trailer with value mcuboot_image_trailer_init as final step of image upload
4. device is rebooted via "mcumgr reboot" or external reset
5. b0 checks if one or both mcuboot slots have a trailer.magic, if not normal boot proceeds
6. b0 updates mcuboot_image_trailer.status appropriately:
   From TESTING to BOOTING (if any TESTING)
   From BOOTING to INACTIVE (if any BOOTING)
7. b0 gives a slot 1st priority if its trailer.status=TESTING
   otherwise it prioritizes trailer.status=ACTIVE
8. mcuboot changes trailer.status of booted image to ACTIVE if confirmed by "mcumgr image confirm"
   and simultaneously marks the not-booted image as INACTIVE

NOTE: if any image fails to validate, the other bank will still be attempted as well.
Hence b0 can "fallback" to an image otherwise marked as INACTIVE.

e8bfecd manifest: mcuboot=v1.9.99-ncs3-mozart-v2, mcuboot update via mcumgr
95d68eb samples: change CUSTOM_CHILD_IMAGE to run hello_world
aa3fbc5 cmake: add support for custom child images
32f188d west.yml: use AudioStreamingPlatform mcuboot fork, v1.9.99-ncs3-mozart