usb/gadget: fastboot: add eMMC support for flash command

- add support for 'fastboot flash' command for eMMC devices

Signed-off-by: Steve Rae <srae@broadcom.com>

include/fb_mmc.h

@@ -0,0 +1,8 @@
+/*
+ * Copyright 2014 Broadcom Corporation.
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+void fb_mmc_flash_write(const char *cmd, void *download_buffer,
+			unsigned int download_bytes, char *response);