qt5.inc: allow to set the package arch globally

qt5.inc: allow to set the package arch globally

If Qt5 (qtbase) is machine specific, then all packages depending
on qtbase will also be machine specific, because the (initial)
qtbase configuration becomes part of Qt5/qmake, thus affecting
builds of packages built against this specific version of qtbase.

Since it is not feasible to add PACKAGE_ARCH=... to each and every
recipe that is built using qmake and depends on Qt5, we introduce
a new variable, QT_PACKAGES_ARCH, that can be set by the machine
configuration as needed.

We have to be careful though to only change the architecture of the
resulting packages if we're not compiling a 'native' package.

(From meta-qt5 rev: 57709a4ee2213a3c352b9cedce1d4b16a1b35042)

Signed-off-by: André Draszik <andre.draszik@linaro.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>