Edit: see June' much better answer :)
I think what it means here is "Hurray, Lunch!"
It's not an idiom I know, but it appears in a couple of songs -
"Roll On, Columbia" celebrates the Columbia river and tells it to "roll (that is, "flow") on."
"Roll On 18 Wheeler" celebrates an eighteen wheel truck/lorry.
Why Neil used "roll on" about lunch, I don't know.