Kconfig: Expose gnrc/ipv6 configurations

[PeterKietzmann]

Contribution description

This PR moves the configuration macros of gnrc_ipv6 found here to the CONFIG_ namespace, and exposes them to Kconfig.

As agreed offline with @leandrolanzieri I did not include the thread stacksize and priority parameters because these should be defined by the platform, if not changed manually. We might need a better solution to this in future.

In the configuration header file, some defines relate to the FIB & RPL (see here). I think this should changed together with adding "GNRC IPv6 NIB compile configurations" as proposed in #12888.

Testing procedure

• examples/gnrc_networking should still work as usual.
• You should be able to configure the module's parameters via make menuconfig in the test application.

Issues/PRs references

Part of #12888

[leandrolanzieri]

Works as expected. Just two details on the prompts. Please address them and squash directly.

[PeterKietzmann]

Thanks @leandrolanzieri, addressed your comment.

[leandrolanzieri]

@PeterKietzmann thanks! ACK