Fix incorrect successful exit code on create_boot_image failure
Fix incorrect successful exit code on create_boot_image failure

create_boot_image() may fail with ofile_size == 0. In such a case, the
current main() returns exit code 0.

Fix that by simply always returning EXIT_FAILURE on create_boot_image()
failure.

antmicro/zynq-mkbootimage,antmicro/zynq-mkbootimage