synch witch upstream commit 85da97f2

synch up to upstream:
https://github.com/mcu-tools/mcuboot/commit/85da97f2fb4b9c5f53fae5e58d7b4201e33d4688



- introduce MCUBOOT_CPU_IDLE() for support low power single thread
- Allow not working secondary image device and boot form primary device if image available

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