synchronized up to the mcu-tool/mcuboot ad1fb3dd

Synchronized to:
https://github.com/mcu-tools/mcuboot/commit/ad1fb3dde2aaf88b433f6d534c84f843c279fac8



doc: actualize zephyr-rtos port README
Kconfig: prefer swap move if scratch_partition not enabled
Kconfig: Explicitly select CONFIG_CRC for CONFIG_MCUBOOT_SERIAL

- imgtool: add option to export public PEM

merged using GitHub web gui.

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