opencv: switch to out-of-tree builds

This eliminates a few warnings from the configure step and makes sense in general.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>

meta-oe/recipes-support/opencv/opencv_2.4.3.bb

@@ -20,6 +20,10 @@ PR = "r2"
 
 S = "${WORKDIR}/OpenCV-${PV}"
 
+# Do an out-of-tree build
+OECMAKE_SOURCEPATH = "${S}"
+OECMAKE_BUILDPATH = "${WORKDIR}/build-${TARGET_ARCH}"
+
 EXTRA_OECMAKE = "-DPYTHON_NUMPY_INCLUDE_DIR:PATH=${STAGING_LIBDIR}/${PYTHON_DIR}/site-packages/numpy/core/include \
                  -DBUILD_PYTHON_SUPPORT=ON \
                  -DWITH_FFMPEG=ON \