fix(bl2): correct messages with image_id

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

Change-Id: I3f868f3d14c9f19349f0daa8a754179f887339c0
Signed-off-by: Yann Gautier <yann.gautier@st.com>