Blender Git Loki

Fix (unreported) broken 'get system path' in some cases.

There were two issues here actually:
* The hack to allow running Blender directly from the source directory
would just check for a 'release' directory, without actually ensuring it
is release dir from blender source tree, and not some other random
folder.
* GHOST_getSystemDir returns nothing for portable installations, now
we'll then check directly in the blender binary dir in that case.

This fix is more critical in 2.8 branch, where that system path is used
to retrieve new '3D' icons...