buildroot

qt53d, qt5serialbus: move to technology preview section

As explained in
https://wiki.qt.io/New_Features_in_Qt_5.6#Technology_Preview_Modules,
not only qt5quickcontrols2 is a technology preview component in Qt 5.6,
but also qt53d, qt5serialbus.

Signed-off-by: Matt Kraai <kraai@ftbfs.org>
[Thomas: improve commit log with a reference.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>