arm: stm32mp: add config for STM32IMAGE support

By default for trusted boot with TF-A, U-Boot (u-boot-nodtb)
is located in FIP container with its device tree and with
the secure monitor (provided by TF-A or OP-TEE).
The FIP file is loaded by TF-A BL2 and each components is
extracted at the final location.

This patch add CONFIG_STM32MP15x_STM32IMAGE to request the
STM32 image generation for SOC STM32MP15x
when FIP container is not used (u-boot.stm32 is loaded by TF-A
as done previously to keep the backward compatibility).

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>