env: correct overflow check of env_has_init size

Correct the overflow check of the bit-field env_has_init with
the max value of env_location= ENVL_COUNT and no more with the
size of env_locations.

This bit-field is indexed by this enumerate and not by the position in
the env_locations (only used in env_get_location) and the
2 values are different, depending of thea ctivated CONFIG_ENV_IS_ options.

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