Dave, there are several meanings for both these words -- jockie and kail -- but the most likely are 1) a country lad and 2) staple food on the table.

Spelling long ago was variable but the currently preferred spelling for cabbage/broth seems to be kail and my take on this line is that he promises that she will never starve -- there will always be the makings of a basic meal -- but that he will endeavour to provide treats for her when he can (the genteel cakes suited to what he thinks her social station requires).

In other words, he's hoping that the love between them will be sufficient to carry them through any hard times.

Hope this helps give you a handle on this lovely wee song.