While the GNRC_IPV6_NIB_RTR_TIMEOUT is properly handled, it is actually
never fired. Moreover, the router lifetime is set, but never read.
This removes the router lifetime and switches it out for an evtimer
to does the GNRC_IPV6_NIB_RTR_TIMEOUT event.
When there are holes in the NIB (e.g. when entries were removed)
currently the NIB crashes the system due to a failed assertion
(`DEVELHELP` needs to be activated to test this behavior).
This fixes this behavior by making the assertion a check that is always
compiled in.
In the case that GNRC_IPV6_NIB_CONF_ARSM is set but
GNRC_IPV6_NIB_CONF_6LN is not, clang complains about
the function _get_l2addr_from_ipv6 never being used.
I couldn't easily figure out why this passes in Murdock,
but I'm guessing that clang is simply being smarter than
GCC. Can someone comment on whether there is a better fix
for this?
Relates to #6473
_is_reachable is only used when GNRC_IPV6_NIB_CONF_ARSM
is enabled, and as such it must be guarded so that clang
doesn't complain about a unused function in case
GNRC_IPV6_NIB_CONF_ARSM is not set
Similar to #7910
Relates to 6473
Since gcc-7 `Wimplicit-fallthrough` is activated by using `-Wextra`.
This leads to the following problem when compiling `gnrc_networking`:
```
RIOT/sys/net/gnrc/network_layer/ndp/internal/gnrc_ndp_internal.c: In function ‘gnrc_ndp_internal_set_state’:
RIOT/sys/net/gnrc/network_layer/ndp/internal/gnrc_ndp_internal.c:106:15: error: this statement may fall through [-Werror=implicit-fallthrough=]
t = ipv6_iface->reach_time;
~~^~~~~~~~~~~~~~~~~~~~~~~~
RIOT/sys/net/gnrc/network_layer/ndp/internal/gnrc_ndp_internal.c:109:9: note: here
case GNRC_IPV6_NC_STATE_DELAY:
^~~~
```
The fall-through in this code is intentional. There are several ways to
warn the comiler about such intentional fall-throughs, which include
e.g. attributed empty statements (`__attribute__ ((fallthrough));`).
I don't like tis approach however. The best way would probably be to
remove this fall-through from the code. However, to keep the diff
minimal, and since ndp will change in the future, I went for warning
the compiler using comments.
The compiler checks comments for several *fall through* regexs to
decide whether a fallthrough was intentional or not.
You can read more about this gcc option in [1]. A note about
fallthrough comment regexs is at the bottom of this article.
[1] https://developers.redhat.com/blog/2017/03/10/wimplicit-fallthrough-in-gcc-7/
