boost: set up PACKAGECONFIG for boost-python library

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].

[1] https://github.com/bmwcarit/meta-ros/pull/145

(From OE-Core rev: 7568bfdd114597956a1da68746f207ec7f93a48d)

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@oss.bmw-carit.de>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>