image: fit: Align hash output buffers

Hardware-accelerated hash functions require that the input and output
buffers be aligned to the minimum DMA alignment. memalign.h helpfully
provides a macro just for this purpose. It doesn't exist on the host,
but we don't need to be aligned there either.

Fixes: 5dfb5213 ("[new uImage] New uImage low-level API")
Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>