bootutil_public: add function for determine image_ok flag

Added boot_read_image_ok() to public API as reading image_ok
might be used by the chain-loaded application for checking by
image itself its confirmation state.

Additionally extracted common code for determine a trailer flag to
boot_read_flash() function.

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