merge: synchronized with mcutool/mcuboot 1a9c6d84

- based on mcuboot 1.8.0

- documentation tweaks
- bootutil: Fixed boot_read_image_header() error path when flash_area is unavilable
- bootutil: Fixed boot_find_status() leaving flash area open
- bootutil: Fixed possible uninitialized flash_area access in boot_write_status()
- Fixed support for single application with serial recovery

merge using GH web GUI

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