configs: stm32mp1: only support SD card after NOR in bootcmd_stm32mp

In the boot command used in ST boards, bootcmd_stm32mp, only support
the SD card as second stage, where is found the bootfs with DISTRO.

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