USB: gadget: added a saner gadget downloader registration API

Preprocessor definitions and hardcoded implementation selection in
g_dnl core were replaced by a linker list made of (usb_function_name,
bind_callback) pairs.

Signed-off-by: Mateusz Zalega <m.zalega@samsung.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Acked-by: Marek Vasut <marex@denx.de>
diff --git a/common/cmd_thordown.c b/common/cmd_thordown.c
index c4b3511..2dd7509 100644
--- a/common/cmd_thordown.c
+++ b/common/cmd_thordown.c
@@ -22,7 +22,6 @@
 	char *interface = argv[2];
 	char *devstring = argv[3];
 
-	const char *s = "thor";
 	int ret;
 
 	puts("TIZEN \"THOR\" Downloader\n");
@@ -40,7 +39,7 @@
 		goto exit;
 	}
 
-	g_dnl_register(s);
+	g_dnl_register("usb_dnl_thor");
 
 	ret = thor_init();
 	if (ret) {