image-fit: Allow loading FIT image for VxWorks

This adds the check against IH_OS_VXWORKS during FIT image load,
to allow loading FIT image for VxWorks.

Signed-off-by: Lihua Zhao <lihua.zhao@windriver.com>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>