fix(fconf): correct image_id type in messages

As image_id is unsigned, we have to use %u and not %d.
This avoids warning when -Wformat-signedness is enabled.

Change-Id: I292e1639847e69ba79265fc32871c0ad7eebc94e
Signed-off-by: Yann Gautier <yann.gautier@st.com>