Use plat_try_next_boot_source() to look for a backup
partition if available.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Change-Id: I3e3a3ee42c81d0195cad16432754ffa3e9f50e3f
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/159358


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: CIBUILD <smet-aci-builds@lists.codex.cro.st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@st.com>