Merge: synchronized up to https://github.com/mcu-tools/mcuboot/commit/9b97456

Synch up to:
https://github.com/mcu-tools/mcuboot/commit/9b97456eb2



- boot_serial: allow to build when CONFIG_MULTITHREADING=n
- allow to not provide scratch area definition if scratch algorithm is not used.

Merged by GH GUI

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