qt5: Add an option to select default platform

Whithout this path, default platform is automaticaly set (generally
set to "eglfs" as defined in
qt5base/mkspecs/devices/common/linux_device_pre.conf:1). This choice
is not always what the user would like. Thus, user have to manually
appends "-platform <BACKEND>" to command line when running any qt5
application.

This patch allows user to choose default platform explicitly.

[Thomas: slightly improve Config.in help text, from the suggestions
given by Yann E. Morin.]

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>