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>
Guo Ren