adding boost-python support for cv-bridge

The ROS package cv_bridge needs the Boost.Python library. This
commit adds a bbappend file that includes Boost.Python in the
boost recipe using PACKAGECONFIG.
This PACKAGECONFIG for boost is available since commit
7568bfdd114597956a1da68746f207ec7f93a48d
in the openembeddded-core repository [1].

[1] http://cgit.openembedded.org/openembedded-core/commit/?id=7568bfdd114597956a1da68746f207ec7f93a48d

recipes-support/boost/boost_1.54.0.bbappend

+PACKAGECONFIG ?= "python"