libbsd: Make modules configurable via ini files.

[Christian Mauderer]

Hello,

also I had really bad luck with some patches today, I'll post these
for feedback:

This is a part of the changes that I plan in the context of
https://devel.rtems.org/ticket/3351

The current version already allows to enable or disable modules in the
libbsd using a configuration file in the ini format. Although I haven't
implemented that yet that can for example be used to generate a libbsd
with and one without WiFi support by selecting another buildset.

The changes are created in a way that multiple buildsets can be build
with one waf call the same way as multiple versions for different BSPs
could be build with one waf call.

Please note that currently to try the patches one further patch for the
rtems_waf submodule is necessary:
https://lists.rtems.org/pipermail/devel/2018-April/020867.html

Although ticket #3351 will need some more patches, I would say that
these are already usable on their own. It would be great if I could get
some feedback so that I can get them commit-ready.

Future patches will add the possibility to influence features like IPv6
or IPSec and define some more usefull configurations (like a
minimalistic libbsd that has about the same functionality like the
legacy stack, a 'default' one that has the most common functionality and
a 'full' one which can do everything).

Best regards

Christian Mauderer