vconsole-setup: use "@kernel" rather than "kernel" as special string to leave keymap untouched

This is a magic string, and we should avoid stepping into the territory
of normal keymap names with that, given that users can pick names
otherwise freely.

Hence, prefix the name with a special char to avoid any namespace
issues.

Follow-up for: #28660

src/vconsole/vconsole-setup.c

@@ -282,7 +282,7 @@ static int keyboard_load_and_wait(const char *vc, Context *c, bool utf8) {
         map_toggle = context_get_config(c, VC_KEYMAP_TOGGLE);
 
         /* An empty map means kernel map */
-        if (isempty(map) || streq(map, "kernel"))
+        if (isempty(map) || streq(map, "@kernel"))
                 return 0;
 
         args[i++] = KBD_LOADKEYS;