riscv: set default FIT generator script and build target for SPL builds

riscv: set default FIT generator script and build target for SPL builds

Now that we have a generic FIT generator script for RISC-V, set it as
the default. To also build the FIT image by default, set the default
build target to "u-boot.itb" if CONFIG_SPL_LOAD_FIT is enabled.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>