qt5base: new package

This is the beginning of the Qt5 packaging. This commit allows to
build only the qtbase module, which contains QtCore, QtNetwork, QtXml,
QtTest, QtSql and QtConcurrent.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>