kernel-fitimage: Use KERNEL_OUTPUT_DIR where appropriate

We have a specific variable for the path to the boot directory. Use it
instead of open-coding this path.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 725b75e8)
Signed-off-by: Steve Sakoman <steve@sakoman.com>