package/python-autobahn: drop asyncio files for python 2.x to fix pycompile issu

Fixes:
http://autobuild.buildroot.net/results/234/234913f86da45df0708bbe3bf7361169e2398c9f

Autobahn contains optional logic using asyncio, which causes pycompile
issues when running in python 2.x:

pycompile.py --strip-root /srv/storage/autobuild/run/instance-1/output-1/target /srv/storage/autobuild/run/instance-1/output-1/target/usr/lib/python2.7
error:   File "/usr/lib/python2.7/site-packages/autobahn/xbr/_blockchain.py", line 97
    async def get_market_status(self, market_id):
            ^
SyntaxError: invalid syntax

As a workaround, simply drop the unusable files from TARGET_DIR if building
for python 2.x.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/python-autobahn/python-autobahn.mk

@@ -11,4 +11,15 @@ PYTHON_AUTOBAHN_LICENSE = MIT
 PYTHON_AUTOBAHN_LICENSE_FILES = LICENSE
 PYTHON_AUTOBAHN_SETUP_TYPE = setuptools
 
+ifeq ($(BR2_PACKAGE_PYTHON),y)
+# only needed/valid for python 3.x
+define PYTHON_AUTOBAHN_RM_PY3_FILES
+	rm -rf $(TARGET_DIR)/usr/lib/python*/site-packages/autobahn/asyncio \
+		$(TARGET_DIR)/usr/lib/python*/site-packages/autobahn/xbr \
+		$(TARGET_DIR)/usr/lib/python*/site-packages/autobahn/twisted/xbr.py
+endef
+
+PYTHON_AUTOBAHN_POST_INSTALL_TARGET_HOOKS += PYTHON_AUTOBAHN_RM_PY3_FILES
+endif
+
 $(eval $(python-package))