Add fixes for webos_qmake5

- Add QT_CONFIGURE_* variables
- Don't run do_generate_qt_config_file until after qt5-native populates
  sysroot

Open-webOS-DCO-1.0-Signed-off-by: Eric Blade <eric.blade@palm.com>

Change-Id: I09dec8d1f84222d4efa0167a51e7981537af7e1c
Reviewed-on: https://g2g.palm.com/1689
Reviewed-by: Eric Blade <eric.blade@palm.com>
Tested-by: Eric Blade <eric.blade@palm.com>
Reviewed-by: Mikko Levonmaa <mikko.levonmaa@palm.com>