configs: stm32f746-disco: Concatenate spl and u-boot binaries

This allows to concatenate spl and u-boot binaries together.
Previously, both binaries has to be flashed separately at the correct
offset (spl at offset 0 and u-boot at offset 0x8000).
With this patch, only one binary is generated (u-boot-with-spl.bin)
and has to be copied in flash at offset 0 using openocd for example
or simply copied in exported mass storage.

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