Fix paths in post install script for PM

This patch fixes the post install script on PM builds by using finddir to determine the user add-ons directory instead of using a hardcoded path. It also works on R1A4 by checking for the presence of B_SYSTEM_NON_PACKAGED directory and if it doesn't find it, it uses the regular B_USER_ADDONS_DIRECTORY. Unfortunately I couldn't check B_USER_NONPACKAGED_ADDONS_DIRECTORY directly because R1A4 seems to define that constant even though it doesn't exist.