modules: introduce MCUBOOT_BOOTUTIL module

Some parts of subsys/dfu/boot code are re-implementations of
what is implemented in the MCUBoot repository.

Mcuboot's repository already provide implementation of function
required for application for interact with the MCUboot.

This patch introduces new MCUBOOT_BOOTUTIL module which covers
common code which is used in the bootloader and the chainnloaded
application.

dfu/boot: use MCUBoot's source code

Module was reworked so it start using MCUBoot's
bootutil_public API instead of copied code.

Reworked boot_is_img_confirmed() used MCUBoot's API
for determine image_ok flag.

mcuboot_shell switchd to use MCUboot's boot_read_swap_state_by_id()
This is MCUBoot function, use it for avoid linking conflict.

test/subsys/mcuboot: fix `test_write_confirm`

dfu/boot library was reworked so it uses MCUboot's bootutil_public
library whenever it can.
The library required that image was marked as copy-done before it
can be pending.
This patch adds such mark which fixes the test.

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