gnrc/ipv6_auto_subnets: allow to configure minimal prefix length

[benpicco]

Contribution description

Linux only accepts Router Advertisements for SLAAC if the prefix length is /64.
Add a config option to gnrc_ipv6_auto_subnets to create smaller subnets if necessary.

Testing procedure

A downstream Linux host gets a prefix from the upstream RIOT router with gnrc_ipv6_auto_subnets:

104: usb0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN group default qlen 1000
    link/ether 66:81:20:f9:aa:94 brd ff:ff:ff:ff:ff:ff
    inet 169.254.226.86/16 brd 169.254.255.255 scope global noprefixroute usb0
       valid_lft forever preferred_lft forever
    inet6 fd00:0:c000:0:e46:895a:43bc:f080/64 scope global dynamic mngtmpaddr noprefixroute 
       valid_lft 72155sec preferred_lft 155sec
    inet6 fe80::650a:e7fd:8b57:c7e8/64 scope link 
       valid_lft forever preferred_lft forever

fd00:0:c000::/64 dev usb0 proto ra metric 304 pref medium

Issues/PRs references

[fabian18]

We probably will have to use may_be_zero more often.

[riot-ci]

Murdock results

✔️ PASSED

2a76911 gnrc/ipv6_auto_subnets: allow to configure minimal prefix length

Success	Failures	Total	Runtime
6946	0	6946	11m:54s

Artifacts

• Documentation preview

[benpicco]

bors merge

[bors]

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• Murdock
• static-tests
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