plat/st: add panic when the boot interface is not supported

Add panic when the boot interface is not supported to stop BL2 execution.

print_boot_device() already raise panic for unknown boot interface.
So this case only occurs when known boot interface is not supported,
removed by compilation flag.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Change-Id: Ib57017183b31d8d79d9f180d76d592291cad1fe5