net/gnrc/netif: set 6LN flag for ethernet if gnrc_sixloenc is used

[benpicco]

Contribution description

AFAIU the gnrc_sixloenc enables 6lo encoding on ethernet.
This can be useful for very slow busses, where we want to reduce traffic as much as possible.

I noticed that GNRC would still perform duplicate address detection on startup - this was because the 6LN flag was not set yet.

Testing procedure

I enabled

USEMODULE += gnrc_sixloenc
CFLAGS += -DCONFIG_GNRC_IPV6_NIB_SLAAC=1
CFLAGS += -DCONFIG_GNRC_IPV6_NIB_NO_RTR_SOL=1
CFLAGS += -DCONFIG_GNRC_IPV6_NIB_ARSM=0

and now get serene silence on the bus if there is no traffic.

Issues/PRs references

[riot-ci]

Murdock results

✔️ PASSED

bc2c95c net/gnrc/netif: set 6LN flag for ethernet if gnrc_sixloenc is used

Success	Failures	Total	Runtime
8082	0	8082	17m:43s

Artifacts

• Documentation preview

[fabian18]

Looking at places where gnrc_sixloenc is used it makes sense to add this change.