stm32mp1: add legacy STM32 image load

STM32MP_USE_STM32IMAGE is now used to distinguished the 2 possible
configurations.
If STM32MP_USE_STM32IMAGE is set, the legacy loading scheme is used. It
implies a single binary containing BL2, BL32 and a shared device tree
blob in case sp_min is used. The legacy loading scheme can also manage
the loading of OP-TEE.
If STM32MP_USE_STM32IMAGE is not set, then FIP is used, and it implies
FCONF as well.

Change-Id: Ibf93982e6be2f551d65094c985eaf5a6bd2ab788
Signed-off-by: Yann Gautier <yann.gautier@st.com>