bootutil: Little rework of boot_find_status

In rework:
  - size has been removed from array, as it has been making array
    at least of two elements, even if scratch partition not used;
  - some variables have been localized and some removed;
  - no longer returning rc codes from flash_area_ functions on
    failure, just single point of failure-exit - it made no sense
    anyway as we can not recover from flash failure;
  - flash area pointer is now NULLified on failure, or when
    status not found.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>