gnrc_netif_conf: fix auto-6ctx switch

[miri64]

Contribution description

When configuring a boolean via Kconfig and setting it to n, the result is that the symbol will not be defined as a preprocessor macro. As a result, setting CONFIG_GNRC_NETIF_IPV6_BR_AUTO_6CTX to n has no effect, as

#ifndef CONFIG_GNRC_NETIF_IPV6_BR_AUTO_6CTX
#define CONFIG_GNRC_NETIF_IPV6_BR_AUTO_6CTX   1
#endif

(as was pointed out in #17678 (comment)). Without changing the name to a negative, the only other valid option is to check if gnrc_netif was configured via Kconfig in addition to the normal #ifndef. This PR applies that change.

Testing procedure

Once a global address is configured with

BOARD=native RIOT_CONFIG_GNRC_NETIF_IPV6_BR_AUTO_6CTX=n RIOT_CONFIG_KCONFIG_USEMODULE_GNRC_NETIF=y make -C examples/gnrc_border_router flash -j term

the table returned by 6ctx should remain empty.

Issues/PRs references

Follow-up on #17678

[leandrolanzieri]

I think the test proceeding is missing a SHOULD_RUN_KCONFIG=1 or a previous Kconfig run to actually run Kconfig and apply the command line options.

BOARD=native RIOT_CONFIG_GNRC_NETIF_IPV6_BR_AUTO_6CTX=n RIOT_CONFIG_KCONFIG_USEMODULE_GNRC_NETIF=y SHOULD_RUN_KCONFIG=1 make -C examples/gnrc_border_router -j

I tested by adding a check in gnrc_netif.c:

#if IS_ACTIVE(CONFIG_GNRC_NETIF_IPV6_BR_AUTO_6CTX)
#error "Compression context"
#else
#error "No compression context"
#endif

With this PR the option is not overwritten by the header file when configuring with Kconfig.

[miri64]

I think the test proceeding is missing a SHOULD_RUN_KCONFIG=1 or a previous Kconfig run to actually run Kconfig and apply the command line options.

IIRC with the RIOT_CONFIG environment variables this is implied.

[leandrolanzieri]

I think the test proceeding is missing a SHOULD_RUN_KCONFIG=1 or a previous Kconfig run to actually run Kconfig and apply the command line options.

IIRC with the RIOT_CONFIG environment variables this is implied.

The example is explicitly disabling it

RIOT/examples/gnrc_border_router/Makefile

Lines 112 to 114 in 9e283b9

	# As there is an 'Kconfig' we want to explicitly disable Kconfig by setting
	# the variable to empty
	SHOULD_RUN_KCONFIG ?=

[miri64]

Backport provided in #18375