refactor(st): rename plat_set_image_source

The function is only used in this file and is static, no need to have
plat_ prefix. And as it is used only in case of FWU, when looking in
metadata, add it in the function name.

Suggested-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I925c9c517216cf93bd74308c280c0f22c7734490
1 file changed