Allow qmake to find features when using mkspecs in nested dirs

Allow qmake to find features when using mkspecs in nested dirs

Commit 8e5eb1bddcfc introduced the assumtion that mkspecs are
immediately below the mkspecs directory itself. This is not true for
e.g. unsupported/blackberry-armv7le-qcc.

This commit restores qmake's ability to find the "root" of the mkspecs
collection no matter how deeply the actual mkspecs are nested.

Task-number: QTBUG-24665
Change-Id: I98faaf8e6ae7b8524277aea6c17e685e507e37b3
Reviewed-by: Sean Harmer <sh@theharmers.co.uk>
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>