python-sip: new package

Based on http://lists.busybox.net/pipermail/buildroot/2013-October/080830.html



[Thomas:
 - fix license information: it is a "or" between the SIP license,
   GPLv2 and GPLv3
 - the target version of python-sip does *not* need the host version
   of python-sip, it can build perfectly fine without it. python-qt
   will need host-python-sip, but there's no need to have this
   dependency between python-sip and host-python-sip.
 - add support for Python 3, since the package could also be enabled
   for Python 3.
 - add <pkg>_BUILD_CMDS for host and target variants, so that the
   build does not take place when doing the installation.
 - use HOST_MAKE_ENV and TARGET_MAKE_ENV where appropriate.
 - fix indentation.
 - propagate the thread and mmu dependencies from Qt.
 - extend the Config.in help text.]

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Cc: Sergey Kostanbaev <sergey.kostanbaev@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>