gnrc_ipv6_nib: make configurable via Kconfig again [backport 2022.07]

[miri64]

Backport of #18360

Contribution description

Some CFLAGS snuck into the Makefile.dep of GNRC which makes it impossible to configure gnrc_ipv6_nib via Kconfig. As an added bonus I added the symbol CONFIG_GNRC_IPV6_NIB_ADD_RIO_IN_LAST_RA which was also induced via CFLAGS in GNRC's Makefile.dep to the Kconfig configuration.

Testing procedure

I noticed this by increasing the size of the forwarding table for the border router (e.g. to allow for large networks with RPL storing mode). I added examples/gnrc_border_router/app.config with content

CONFIG_KCONFIG_USEMODULE_GNRC_IPV6_NIB=y
CONFIG_GNRC_IPV6_NIB_NUMOF=50
CONFIG_GNRC_IPV6_NIB_OFFL_NUMOF=50

Trying to make -C examples/gnrc_border_router will get you a bunch of these error messages in current master:

make[4]: *** [/home/mlenders/Repositories/RIOT-OS/RIOT/Makefile.base:146: /home/mlenders/Repositories/RIOT-OS/RIOT/examples/gnrc_border_router/bin/native/gnrc_netif/gnrc_netif.o] Error 1
make[3]: *** [/home/mlenders/Repositories/RIOT-OS/RIOT/Makefile.base:31: ALL--/home/mlenders/Repositories/RIOT-OS/RIOT/sys/net/gnrc/netif] Error 2
In file included from <command-line>:
/home/mlenders/Repositories/RIOT-OS/RIOT/examples/gnrc_border_router/bin/native/generated/autoconf.h:200: error: "CONFIG_GNRC_IPV6_NIB_ADV_ROUTER" redefined [-Werror]
  200 | #define CONFIG_GNRC_IPV6_NIB_ADV_ROUTER 1
      | 
<command-line>: note: this is the location of the previous definition
In file included from <command-line>:
/home/mlenders/Repositories/RIOT-OS/RIOT/examples/gnrc_border_router/bin/native/riotbuild/riotbuild.h:6: error: "CONFIG_GNRC_IPV6_NIB_ADV_ROUTER" redefined [-Werror]
    6 | #define CONFIG_GNRC_IPV6_NIB_ADV_ROUTER 0
      | 
/home/mlenders/Repositories/RIOT-OS/RIOT/examples/gnrc_border_router/bin/native/generated/autoconf.h:200: note: this is the location of the previous definition
  200 | #define CONFIG_GNRC_IPV6_NIB_ADV_ROUTER 1
      | 
cc1: all warnings being treated as errors

This PR fixes that.

Issues/PRs references

None