adjusting dependencies of native recipes

To compile nodelet-topic-tools, we must provide a working
dynamic-reconfigure in the native sysroot. This requires
some adjustments to the native versions of the required
python modules.

In particular, the following tasks ran successfully for this
commit:

This command tests compiling rospack-native:
rm bitbake.lock cache/ sstate-cache/ tmp-eglibc/ -rf && bitbake rospack-native

This command tests compiling roslib-native:
rm bitbake.lock cache/ sstate-cache/ tmp-eglibc/ -rf && bitbake roslib-native

As nodelet-topic-tools uses rospack-native and roslib-native to
some extent (dependency: rospack-native -> roslib-native ->
dynamic-reconfigure-native -> nodelet-topic-tools), this command
tests running rospack-native and roslib-native:
rm bitbake.lock cache/ sstate-cache/ tmp-eglibc/ -rf && bitbake nodelet-topic-tools

This command tests compiling rospack and roslib:
rm bitbake.lock cache/ sstate-cache/ tmp-eglibc/ -rf && bitbake rospack
rm bitbake.lock cache/ sstate-cache/ tmp-eglibc/ -rf && bitbake roslib

All these command run with this commit without errors, and
future commits to these files should keep them run without
errors or justify with better tests.

A minor open issue related to rospack is issue #116
(cf. https://github.com/bmwcarit/meta-ros/issues/116).

This commit incorporates feedback from Stefan Herbrechtsmeier.