refactor(plat/st): check boot device only for BL2

The boot device is now checked inside a dedicated rule, that is only
called during BL2 compilation step

Change-Id: Ie7bcd1f166285224b0c042238989a82f7b6105c6
Signed-off-by: Yann Gautier <yann.gautier@st.com>
1 file changed