sys/net/gnrc/netif: Make aac_mode a flag field

[JKRhb]

Contribution description

This PR is another spin-off from #16228. Its changes let the the aac_mode field of gnrc_netif_ipv6_t be treated as flag field.
The members of the net_gnrc_netif_aac enum are updated accordingly. This way, it is possible for a net interface to use (for instance) both SLAAC and DHCPv6 for auto-adress configuration.

Testing procedure

As this is curently more or less a refactoring, you can use the existing gnrc and unit tests should which should still be passing.

Issues/PRs references

[JKRhb]

Hmm, one test has failed because of unsufficient ROM (4 bytes overflow). Should the board (stm32mp157c-dk2) be added to the BOARD_INSUFFICIENT_MEMORY list or is there maybe a way to fix this?

[benpicco]

I'd say just add it to the list - if it's this tight it's not worth the trouble.
By how much does the ROM consumption increase compared to master? I can't imagine it to be more than a handful of bytes.

[JKRhb]

I'd say just add it to the list - if it's this tight it's not worth the trouble.

Will do :)

By how much does the ROM consumption increase compared to master? I can't imagine it to be more than a handful of bytes.

If I am not mistaken, it should be 8 bytes (before, after). So pretty close :/

[benpicco]

Looks good to me - @miri64 any objections?

[miri64]

Nope, was my proposal after all :-), let's gooooo.