host-libxml2: Prefer python2 when python3 is also installed

host-libxml2: Prefer python2 when python3 is also installed

Fixes mesa3d build errors:
http://autobuild.buildroot.net/results/d10/d105a0b3ca11fad34f9a2dae0dae9bd041d918a6/
http://autobuild.buildroot.net/results/d71/d7121443715024d15a66ff1abf1261803c10cd35/
http://autobuild.buildroot.net/results/ce6/ce64164d76972f82acab277afc9c95a876c6433e/

checking for python2... python2
checking python2 module: libxml2... no
configure: error: failed to find required module libxml2
make: *** [/home/test/test/1/output/build/mesa3d-10.0.4/.stamp_configured] Error 1

mesa3d needs python2 bindings installed by libxml2. To enforce their build
option BR2_PACKAGE_HOST_LIBXML2_PYTHON is used exclusively by mesa3d so this
patch does not affect other packages.

Without this patch host-libxml2 installs the python bindings in
$(HOST_DIR)/usr/lib/$(PYTHON3_VERSION_MAJOR) using this defconfig:

BR2_PACKAGE_MESA3D=y
BR2_PACKAGE_PYTHON3=y

because $(HOST_DIR)/usr/bin/python points to $(HOST_DIR)/usr/bin/python3:

Quote from host-libxml2 configure log:
Found python in /home/fli4l/br2/buildroot/output/host/usr/bin/python
Found Python version 3.4

HOST_$(PACKAGE)_NEEDS_HOST_PYTHON does not work here because libxml2 does
not use the python-package infrastructure.

libxml2-python2.patch extends the python detection code in host-libxml2 to
first look for python2, to keep the patch small I did not update the indentions:

Found python2 in /home/fli4l/br2/buildroot/output/host/usr/bin/python2
Found Python version 2.7

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Tested-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>