configs: sandbox: activate env in ext4 support

Activate ENV in EXT4 support in sandbox.

The sandbox behavior don't change; the default environment with
the nowhere backend (CONFIG_ENV_IS_NOWHERE)is still used:
the weak function env_get_location() return ENVL_NOWHERE for priority 0.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>