Enum parsing is a little broken

[hamishwillee]

So the way this works is that when you do merge_enums() the x.enums for each xml file gets modified. If the file is the first to have an enum it gets to keep it and all the duplicate enum values are merged in. If the enum is "new" then it gets added to a list of new enums that get assigned back to the xml file. In other words, each XML file is stripped of any enums that are already merged into the top level.

What this means is that the top level file works fine - it has access to all enums it defined, including anything that is merged. However if you were to include mavlink.h from a dialect you would find it missing any enums that were declared in the parent, including any values that were in the XML.

This is different from other things like messages, where we use #defines to make sure that at each level you get what is underneath. I think this is a bug - i.e. that we make the assumption that when you generate a dialect you can use mavlink.h from any included dialect and use it as though you had build that included dialect directly.

Does this make sense - do we all agree it is a bug?

What it also means is that if you generate multiple dialect files the order matters - since you can remove the enums from a lower level

If so, any thoughts on fixing?