stm32mp1: configs: Make all boot devices in distro boot conditional

Not all systems have all the boot devices enabled, e.g. not all systems
have MTD devices and thus do not enable UBI. Make all the boot devices
in the distro bootcmd conditional to avoid failures.

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