fastboot: Implement NAND backend

So far the fastboot code was only supporting MMC-backed devices for its
flashing operations (flash and erase).

Add a storage backend for NAND-backed devices.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

include/fb_nand.h

@@ -0,0 +1,11 @@
+/*
+ * Copyright 2014 Broadcom Corporation.
+ * Copyright 2015 Free Electrons.
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+void fb_nand_flash_write(const char *cmd, unsigned int session_id,
+			 void *download_buffer, unsigned int download_bytes,
+			 char *response);
+void fb_nand_erase(const char *cmd, char *response);