Merge: synhronize up to https://github.com/mcu-tools/mcuboot/commit/680ed07e42f

Synch up to:
https://github.com/mcu-tools/mcuboot/commit/680ed07e42fb67cf855360ff2d0ec00a6bf38104



* Added AES256 support for image encryption
* zephyr: serial: Remove unnecessary call to irq_unlock 
* boot_serial: Port encoding to use cddl-gen
* bootutil_public: Make boot_read_swap_state declaration public
* zephyr: allow to build nRF52840 targets in zephyr-rtos CI

Merged by GH GUI

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>