fastboot: add support for continue command

The fastboot continue command is defined to exit fastboot and continue
autoboot. This commit implements the continue command and the exiting of
fastboot only. Subsequent u-boot commands can be processed after exiting
fastboot. Autoboot should implement a boot script such as "fastboot; mmc
read <...>; bootm" to fully implement the fastboot continue function.

Signed-off-by: Rob Herring <robh@kernel.org>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>
[TestHW: Exynos4412-Trats2]
diff --git a/common/cmd_fastboot.c b/common/cmd_fastboot.c
index 909616d..b72f4f3 100644
--- a/common/cmd_fastboot.c
+++ b/common/cmd_fastboot.c
@@ -15,17 +15,21 @@
 {
 	int ret;
 
+	g_dnl_clear_detach();
 	ret = g_dnl_register("usb_dnl_fastboot");
 	if (ret)
 		return ret;
 
 	while (1) {
+		if (g_dnl_detach())
+			break;
 		if (ctrlc())
 			break;
 		usb_gadget_handle_interrupts();
 	}
 
 	g_dnl_unregister();
+	g_dnl_clear_detach();
 	return CMD_RET_SUCCESS;
 }