qttools: make qtwebkit a PACKAGECONFIG and disable by default

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
qtimageformats: Make the dependencies deterministic

* jasper and libwebp were autodetected from sysroot

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>