EPOCROOT needs to be usable in qt.conf also when devices.xml is used to determine Symbian SDK

Currently if you want to use EPOCROOT in qt.conf, you are limited to using EPOCROOT environment variable, while qmake and configure support resolving epocroot via devices.xml. All use cases where EPOCROOT needs to be resolved should use the same logic. It would also allow user to have EPOCROOT with (windows style) or without (linux style) the ending slash.