boost: set up PACKAGECONFIG for boost-python library
Until now, the comment in the boost.inc file suggested to enable
the boost-python library by uncommenting some lines in that file.
Using the new PACKAGECONFIG feature, boost-python can now be added
optionally without need to modify the file or copying those lines
into a bbappend file.
Furthermore, we obtain the python version by inheriting python-dir
instead of fixing the python version in this file.
This commit is motivated by the need in the meta-ros layer, as
discussed in the issue #145 of the meta-ros issue tracker [1].
