boot-directdisk: Use kernel from DEPLOY_DIR_IMAGE, not sysroot
After the recent kernel changes, this avoids failures like:
DEBUG: Executing shell function build_boot_dd
install: cannot stat '/home/pokybuild/yocto-autobuilder/yocto-worker/build-appliance/build/build/tmp/sysroots/qemux86-64/usr/src/kernel/bzImage': No such file or directory
WARNING: exit code 1 from a shell command.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
