sys/shell_commands: convert to SHELL_COMMAND()

[maribu]

Contribution description

This drops the manually managed "shell command registry" and uses the XFA based SHELL_COMMAND() mechanism for registration instead.

Open ToDos

Adding a static in front of the handler function should now be possible in most cases, except when the command handler is actually used externally. I'd rather do this in a follow up and get his in as quickly and smoothly as possible to avoid merge conflicts.

Testing procedure

Except for alphabetical ordering, the shell command should still all registered as before.

Issues/PRs references

Same as #16095 but already moved the SHELL_COMMAND() registration next to the implementation.

[maribu]

If you wonder why I dropped the abuse of the ifconfig shell command being called from C without ever running the shell from examples/nanocoap_server: Link time garbage collection doesn't work with XFA and all the shell command registration as well as their implementation are now linked in. I just copy-pasted a few lines from examples/gnrc_minimal to print the obtained addresses instead, which should be even small than it was in master.

Also, it is IMO super confusing to see shell command output without the shell. I assumed the application to be crashed when I was unable to invoke shell commands via serial until I realized that no shell is actually running.

[maribu]

How did this ever work?

RIOT/sys/shell/commands/shell_commands.c

Lines 109 to 112 in 6e41c68

	#ifdef MODULE_OPENWSN
	extern int _openwsn_ifconfig(int argc, char **argv);
	extern int _openwsn_handler(int argc, char **argv);
	#endif

RIOT/sys/shell/commands/sc_openwsn.c

Lines 96 to 99 in 6e41c68

	int _openwsn_ifconfig(char *arg)
	{
	(void)arg;

[fjmolinas]

How did this ever work?

RIOT/sys/shell/commands/shell_commands.c

Lines 109 to 112 in 6e41c68

	#ifdef MODULE_OPENWSN
	extern int _openwsn_ifconfig(int argc, char **argv);
	extern int _openwsn_handler(int argc, char **argv);
	#endif

RIOT/sys/shell/commands/sc_openwsn.c

Lines 96 to 99 in 6e41c68

	int _openwsn_ifconfig(char *arg)
	{
	(void)arg;

No idea 😕, sorry about that

[miri64]

How did this ever work?

[maribu]

rebased to solve merge conflict

[maribu]

All green now 🎉

[maribu]

Thanks for the review!