uboot: Add the missing disk write operation support

uboot_disk_write() is currently lacking the write support
to storage devices because, historically, those devices did not
implement block_write() in U-Boot.

The solution has been tested using a patched U-Boot loading
and booting GRUB in a QEMU vexpress-a9 environment.
The disk write operations were triggered with GRUB's save_env
command.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/kern/uboot/uboot.c

@@ -243,6 +243,22 @@ grub_uboot_dev_read (struct device_info *dev, void *buf, grub_size_t blocks,
   return retval;
 }
 
+int
+grub_uboot_dev_write (struct device_info *dev, const void *buf,
+		      grub_size_t blocks, grub_uint32_t start)
+{
+  int retval;
+
+  if (!OPEN_DEV (dev))
+    return -1;
+
+  if (!grub_uboot_syscall (API_DEV_WRITE, &retval, dev, buf,
+			   &blocks, &start))
+    return -1;
+
+  return retval;
+}
+
 int
 grub_uboot_dev_recv (struct device_info *dev, void *buf,
 		     int size, int *real_size)