pcsx2: use PCSX2 uppercase for XDG_HOME_CONFIG default value

This way it will always be the same.

Fix issues #381

Note: potentially we might need to move your previous directory.
mv $HOME/.config/pcsx2 $HOME/.config/PCSX2
Merge pull request #387 from PCSX2/XDG_HOME_CONFIG-use-PCSX2-issue-381

pcsx2: use PCSX2 uppercase for XDG_HOME_CONFIG default value