kernel: Note that the image must be uncompressed (raspberrypi#831)

Image.gz is not supported. Note that.

Author: kees

configuration/config-txt/boot.md

@@ -31,7 +31,7 @@ Set the `disable_commandline_tags` command to `1` to stop `start.elf` from filli
 
 ## kernel
 
-`kernel` is the alternative filename on the boot partition to use when loading the kernel. The default value on the Pi 1, Pi Zero, and Compute Module is `kernel.img`, and on the Pi 2, Pi 3, and Compute Module 3 it is `kernel7.img`. If `kernel8.img` is present on the Pi 3 or Compute Module 3, it will be loaded in preference and entered in 64-bit mode.
+`kernel` is the alternative filename on the boot partition to use when loading the kernel. The default value on the Pi 1, Pi Zero, and Compute Module is `kernel.img`, and on the Pi 2, Pi 3, and Compute Module 3 it is `kernel7.img`. If `kernel8.img` is present on the Pi 3 or Compute Module 3, it will be loaded in preference and entered in 64-bit mode. **NOTE**: This must be an uncompressed kernel image file.
 
 ## kernel_address