Revert "common: spl_fit: Default to IH_OS_U_BOOT if FIT_IMAGE_TINY enabled"

Commit cf8dcc5d ("common: spl_fit: Default to IH_OS_U_BOOT if
FIT_IMAGE_TINY enabled") is not correct, it will append fdt to each loadable
image. Actually when using TINY FIT, the first loadable image is thought as
u-boot and already have fdt appended.

Signed-off-by: Ye Li <ye.li@nxp.com>
Tested-by: Fabio Estevam <festevam@gmail.com>