qt5base: add rPi EGL glue code

When the rpi-userland package is selected, assume we are targetting
the Raspberry Pi, and add the right platform glue code when building
the Qt5 EGLFS plugin.

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>