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_dfu.c b/common/cmd_dfu.c
index 5547678..a03538d 100644
--- a/common/cmd_dfu.c
+++ b/common/cmd_dfu.c
@@ -22,7 +22,6 @@
 	char *interface = argv[2];
 	char *devstring = argv[3];
 
-	char *s = "dfu";
 	int ret, i = 0;
 
 	ret = dfu_init_env_entities(interface, simple_strtoul(devstring,
@@ -38,7 +37,7 @@
 	int controller_index = simple_strtoul(usb_controller, NULL, 0);
 	board_usb_init(controller_index, USB_INIT_DEVICE);
 
-	g_dnl_register(s);
+	g_dnl_register("usb_dnl_dfu");
 	while (1) {
 		if (dfu_reset())
 			/*
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) {
diff --git a/common/cmd_usb_mass_storage.c b/common/cmd_usb_mass_storage.c
index 14a5b6a..d8d9efd 100644
--- a/common/cmd_usb_mass_storage.c
+++ b/common/cmd_usb_mass_storage.c
@@ -40,7 +40,7 @@
 		return CMD_RET_FAILURE;
 	}
 
-	g_dnl_register("ums");
+	g_dnl_register("usb_dnl_ums");
 
 	/* Timeout unit: seconds */
 	int cable_ready_timeout = UMS_CABLE_READY_TIMEOUT;