configs: sandbox: activate command env select and env load

Add support of environment location with the new env command:
'env select' and 'env load'

The ENV backend is selected by priority order
- 0 = "nowhere" (default at boot)
- 1 = "EXT4"

To test EXT4 env support, this backend is selected by name:
> env select EXT4

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