lib/fit: Switch to commonlib/list public API

CB:90961 introduces list_is_empty() function. Use it to improve
readability.

Change-Id: Icffbce9e77b19c3885e0a0466f8d109ec84d70c4
Signed-off-by: Yu-Ping Wu <yupingso@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/91006
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jakub "Kuba" Czapiga <czapiga@google.com>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/lib/fit.c

@@ -396,7 +396,7 @@ static int fit_update_compat(struct fit_config_node *config)
 		}
 
 		/* FDT overlays are not supported in legacy FIT images. */
-		if (config->overlays.next) {
+		if (!list_is_empty(&config->overlays)) {
 			printk(BIOS_ERR, "config %s has overlay but no compat!\n",
 			       config->name);
 			return -1;