Add --extend-define that allows extending list defines

This is commonly used for e.g. extra_rpms, if you want to pass in an original list of rpms from somewhere, but then extend it.

The behavior is:

• if the key is unset before, we initialize it with a list
• if the value is a list, we join the list to the old list
• if the value is not a list we append the item to the old list