kernel-yocto.bbclass: fix a wrong inter-task dependency

do_kernel_checkout and do_symlink_kernsrc are both modifying ${S}, they
could conflict with eacher other, move do_kernel_checkout after
do_symlink_kernsrc does fix that.

Signed-off-by: Ming Liu <liu.ming@toradex.com>
Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>