configs: stm32mp15: activate CMD_ELF

Activate CONFIG_CMD_ELF to accept the command "bootelf".

This patch simplifies the file stm32mp defconfig, as we have
no reason to deactivate this command.

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