gnrc_netif: make auto-config of compression context optional

[miri64]

Contribution description

For a set of my currently ongoing experiments it is important that the 6LoWPAN header stays roughly the same over all hops. With global addresses this can easily be achieved by not configuring a compression context. However, when using a border router with UHCP or DHCPv6, the compression context will be configured automatically and when you try to delete it, it is most often to late, as the prefix and the compression contexts are already disseminated throughout the network (and any further auto config message for the prefix might re-add it).

This change makes the auto-configuration of the compression context by setting the Kconfig variable CONFIG_GNRC_NETIF_IPV6_DO_NOT_COMP_PREFIX to false.

Testing procedure

Flash and run the examples/gnrc_border_router twice: Once as is, and once with the new option set (e.g. by using make menuconfig. Once a global prefix is configured (use ifconfig to check) use 6ctx to check: for the untouched binary, 6ctx should show the prefix as compression context 0, for when CONFIG_GNRC_NETIF_IPV6_DO_NOT_COMP_PREFIX=y the table should be empty.

Issues/PRs references

None

[benpicco]

I think #17594 does the same, it will just allow to completely disable the module instead.

[miri64]

I think #17594 does the same, it will just allow to completely disable the module instead.

You still might want to configure compression manually, so I don't think it is the same.

[benpicco]

Please squash

[miri64]

Squashed