pkg/lwip: fix dependencies when LWIP IPv4 and LWIP_DHCP is used

[krzysztof-cabaj]

Contribution description

When LWIP IPv4 and LWIP DHCP is used, in file build/pkg/lwip/src/include/lwip/acd.h (Address Conflict Detection)
definition of struct etharp_hdr is needed. This definition is available in the module netdev_eth.

In link @mguetschow reported that previous solution from PR #21342 did not work for nrf52840dk.

This PR is some "dirty-hack" to solve this problem.

TODO:
Find the root cause of the problem. PR #17174 and PR #17162 could be a good starting point.

Testing procedure

Without this PR on current master LWIP_IPV4=1 make -C examples/networking/coap/gcoap_dtls BOARD=nrf52840dk ends with error:

"make" -C /data/RIOT/RIOT/build/pkg/lwip/src/api -f /data/RIOT/RIOT/Makefile.base MODULE=lwip_api
In file included from /data/RIOT/RIOT/build/pkg/lwip/src/include/lwip/dhcp.h:49,
                 from /data/RIOT/RIOT/build/pkg/lwip/src/include/lwip/netifapi.h:41,
                 from /data/RIOT/RIOT/build/pkg/lwip/src/api/netifapi.c:46:
/data/RIOT/RIOT/build/pkg/lwip/src/include/lwip/acd.h:97:48: error: 'struct etharp_hdr' declared inside parameter list will not be visible outside of this definition or declaration [-Werror]
   97 | void acd_arp_reply(struct netif *netif, struct etharp_hdr *hdr);
      |                                                ^~~~~~~~~~

With this PR, everything works well.
I tested this for nrf52840dk (lack of Ethernet interface but radio interface), nucleo-f439zi (with Ethernet interface), native (lack of any network interface).

Issues/PRs references

Fixes PR #21342

[riot-ci]

Murdock results

✔️ PASSED

6be2534 pkg/lwip: fix dependencies when LWIP IPv4 and LWIP_DHCP is used

Success	Failures	Total	Runtime
10930	0	10931	09m:24s

Artifacts

• Documentation preview

[mguetschow]

Thanks for looking into this! To be honest, I'm not a big fan of this hack, as it seems to only provide a superficial fix at the cost of modeling a wrong netdev during dependency resolution.

While trying to figure out the root cause, I found

RIOT/pkg/lwip/include/lwipopts.h

Lines 35 to 39 in 989b85b

	#if defined(MODULE_LWIP_ARP) && defined(MODULE_LWIP_ETHERNET)
	#define LWIP_ARP 1
	#else /* MODULE_LWIP_ARP */
	#define LWIP_ARP 0
	#endif /* MODULE_LWIP_ARP */

which was added with #17174

It seems to me that the issue #17162 mentioned there is closer to the root cause.

I still think we should merge this now as it improves things, but I'd ask you to add a TODO note to the comment pointing to the issue above and stating that this should be fixed more fundamentally.