feat(fconf): add a helper to get image index

A new function dyn_cfg_dtb_info_get_index() is created to get the index
of the given image config_id in the dtb_infos pool.
This allows checking if an image with a specific ID is in the FIP.

Change-Id: Ib300ed08e5b8a683dc7980a90221c305fb3f457d
Signed-off-by: Yann Gautier <yann.gautier@st.com>
2 files changed