board/csky: fixup gdb instructions in readme.txt

The gdbinit file assumes the .dtb file is in the current directory, so
let's start gdb from output/images.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
[Thomas: improved commit description.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

board/csky/readme.txt

@@ -83,5 +83,6 @@ Run
 
   Setup the Console with the rate 115200/8-N-1.
 
-  $ ./output/host/usr/bin/csky-linux-gdb -x board/csky/gx6605s/gdbinit output/images/vmlinux
+  $ cd output/images
+  $ ../host/usr/bin/csky-linux-gdb -x ../../board/csky/gx6605s/gdbinit vmlinux