Previously only mesh node info is supported to be stored
in flash. So when trying to reset the node, we only need
to judge if the BLE_MESH_VALID flag is set.
Currently we support storing both node & Provisioner info
in flash, when trying to erase the node info from flash,
the BLE_MESH_NODE flag will be checked. So we need to set
bt_mesh.flags to 0 when all the erase operations are done.