Bug 966239: Set GECKO_PATH if not defined by config

The load-config script sets GECKO_PATH if it's not defined in a
config file. Similar code in other scripts has been cleaned up.

Signed-off-by: Thomas Zimmermann <tdz@users.sourceforge.net>
Merge pull request #321 from tdz/bug-966239

Bug 966239: Set GECKO_PATH if not defined by config