RFC: Reworked autoupdate script to be generic and add a couple of features.

Stefan (et al),

This is not complete and certainly hasn't had enough testing yet. However I'd like to get general opinion on it.

My primary reason for changing the script is that autoupdate on my rpi simply doesn't work since I map .xbmc/temp to /dev/shm (as I have always done with my OE builds) to reduce disk wear for temporary files (logs, etc...). This obviously causes issues for rpi update since it doesn't have the capacity to store the download in RAM.

Therefore I added the ability for the download path to be overridden, at the moment this can only be achieved via editing repo.conf so is a power user only feature (but it could be moved to user settings).

But having decided to make changes to the script I was slightly put off by there being 2 near identical scripts (meaning such changes need be made twice) when a single script could easily cover both cases. So I've merged the scripts and simply detect the type of install etc..

Anyway any thoughts appreciated.

Adam