stm32mp1: check boot device only for BL2 compilation

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>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/183646


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: CIBUILD <smet-aci-builds@lists.codex.cro.st.com>
Reviewed-by: Lionel DEBIEVE <lionel.debieve@st.com>