qt: fix plugin installation and change plugin path

Since the conversion of Qt to GENTARGETS in
421cda1f, plugin installation could
break in some situations, for example if SQL support was enabled, but
without any SQL driver: the sql plugin directory doesn't exist, but
our qt.mk wanted to copy it.

This patches simplifies the plugin handling a bit, and basically
copies all Qt plugins installed in the $(STAGING_DIR) to the
$(TARGET_DIR), assuming Qt has only built and installed the needed
plugins.

Moreover, instead of installing plugins to usr/plugins, which is a odd
location, we install them in usr/lib/qt/plugins. This requires a small
patch to Qt ./configure script so that even when -hostprefix is used,
the -plugindir option is taken into account.

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