qt5: factor <pkg>_SITE variable

Each qt5 package was defining its own QT5<something>_SITE
variable. Unfortunately, the download location has slightly changed
for Qt 5.0.2, so we would have to change it for all packages. Instead,
we factor the <pkg>_SITE variable as QT5_SITE in package/qt5/qt5.mk,
just like we already do for QT5_VERSION.

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