configs: stm32mp1: remove splashimage and add fdtoverlay_addr_r

Add the variable used by PXE command for fdtoverlays support
since the commit 69076dff ("cmd: pxe: add support for FDT overlays").

Reused the unused "splashimage" address as CONFIG_SPLASH_SOURCE and
CONFIG_VIDEO_LOGO are not activated and U-Boot display the "BACKGROUND"
image found in extlinux.conf to manage splashscreen on stm32mp1 boards.

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