common: spl_fit: Default to IH_OS_U_BOOT if FIT_IMAGE_TINY enabled

If FIT_IMAGE_TINY is enabled, spl_fit_image_get_os returns -ENOTSUPP.
In this case, we should default to IH_OS_U_BOOT not to IH_OS_INVALID.

Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Tested-by: Fabio Estevam <festevam@gmail.com>