stm32mp1: configs: activate CMD_ERASEENV

Activate the command env erase to reset the environment with the command:
> env erase

it is simpler than:
> env default -a
> env save

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