qt5base: add qmake.conf fixup for i.MX platforms

As stated in QTBUG-61156 [1], since Qt5.9 all the Qt apps fail to start
on i.MX platforms.

The reason is that the qmake.conf should apparently now specify which
eglfs backend to use.

It can be seen in the qmake.conf example for i.MX [2].

It has also been integrated into the Yocto meta-freescale layer [3].

Although this issue can be worked around by exporting a variable
(QT_QPA_EGLFS_INTEGRATION=eglfs_viv), this patch aims at providing
a more user-friendly approach by modifying qmake.conf when created.

[1]: https://bugreports.qt.io/browse/QTBUG-61156
[2]: https://github.com/qt/qtbase/blob/5.9/mkspecs/devices/linux-imx6-g%2B%2B/qmake.conf#L35
[3]: https://github.com/Freescale/meta-freescale/blob/master/dynamic-layers/qt5-layer/recipes-qt/qt5/qtbase/0015-Add-eglfs-to-IMX-GPU.patch



[Peter: simplify conditional, add comment explaining why]
Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Tested-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>