Add support for PKG_UPGRADE variable in package scripts
This variable seems to be set to 1 when package is updated. Up to my knowledge we are not doing that (although I haven't looked in to the code).
We should probably introduce it. There is small problem with it that we do not know if it is a new installation or not in location where we generate those variables, we know only that we have to install it. It should not be hard to see if package is already installed but it is not directly accessible in that code location.