ARM: stm32: Enable bootd, iminfo, imxtract on ST defconfig

Enable these standard U-Boot commands for image manipulation and for
starting the default boot command using 'boot' command in U-Boot shell.

Cc: Marek Vasut <marex@denx.de>
Cc: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>