generate_signed_fit_image: Align FIT image to 64-byte boundary

Task: https://phabricator.apertis.org/T8783

Currently, external data is used for the FIT image so it can be aligned properly matching the kernel expectation for the fdt to be 8 bytes (64 bits) aligned.

FSL CAAM crypt module uses DMA, which expects images to be 64 byte aligned. Let's use this alignment so the FIT image matches all the requirements.

Related u-boot MR pkg/u-boot!62 (merged)