Don't look for weston.ini in the current working directory

It's a bit surprising that Weston looks different when launched from the root
of the git repo vs from elsewhere.

But it's also technically a security vulnerability: if I launch it from
a directory like /tmp, it might pick up a weston.ini created by another user,
which could then load modules with arbitrary code. Basically, it's the same
problem as including "." in $PATH.

Signed-off-by: Dima Ryazanov <dima@gmail.com>