zephyr/boot_serial_extension: added hooks to custom image list MGMT

Introduced boot_img_install_stat_hook() hook fuinction for fetch
the image's slot installation status.

The image's slot installation status is custom property.
It's detailed definition depends on user implementation. It is only
defined that the status will be set to 0 if this hook not provides
another value.

Inserted available hook for read image header as well.

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