qt: move host programs and specs in HOST_DIR

The Qt build system installs all host programs (uic, rcc, moc, qmake,
etc.) in the same base directory as the headers and libraries, but
that's not great for Buildroot.

So we now move the host programs and the mkspecs files to the
$(HOST_DIR), and create a qt.conf, which tells qmake where everything
is located.

Tested with Qmake and Cmake as build system for a simple Qt
application.

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