json-rpc: auto-generate xbmc.json/addon.xml

Now that we auto-generate the JSON-RPC API schema during bootstrap/build I figured we could extend that logic to also auto-generate the addon.xml of the xbmc.json extension point which currently needs manual updating with the updated version of the JSON-RPC API (and which I and others forget almost all the time).

I've testd this with jenkins (see http://jenkins.xbmc.org/view/All/job/XBMC-BuildMulti-All/159/) and it works on all platforms but for linux et al. I've pretty much hacked this into the codegenerator.mk and I'm very unhappy with the implementation (specifically the unintuitive path to addon.xml passed into the json-rpc Makefile as a build target) of the $(GENERATED_ADDON_JSON) target in codegenerator.mk so maybe @wsnipex or someone else has a better approach/idea.