qt: convert to the GENTARGETS infrastructure

qt: convert to the GENTARGETS infrastructure

In addition to converting the qt package to the GENTARGETS
infrastructure, this commit also does the following (related) things
in the Qt package:

 * Create a BR2_PACKAGE_QT_CONFIG_FILE option, which can be used to
   pass a custom Qt configuration file, such as the ones found in
   src/corelib/global/qconfig-*.h. This used to be possible, but
   required changing qt.mk directly, which isn't really how we
   configure things in Buildroot.

 * Remove the BR2_PACKAGE_QT_EMB_PLATFORM option, the embedded
   platform name is now computed directly in qt.mk.

 * The QT_CONFIGURE variable, which hosted all ./configure options,
   has been renamed to QT_CONFIGURE_OPTS, for consistency with what we
   do in the AUTOTARGETS infrastructure.

 * The QT_DEP_LIBS variable has been renamed to QT_DEPENDENCIES, so
   that dependencies are properly handled by the GENTARGETS
   infrastructures.

 * The QT_QMAKE_SET macro (used to adjust the path/flags of the
   compiler/linker) has been extended with an additional argument,
   which allows to pass the source directory of Qt.

 * All the installation procedure has been rewritten to fit within the
   GENTARGETS mechanism.

[Peter: fixed minor issues pointed out by Will]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: Will Wagner <willw@carallon.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>