Split the common RISC-V boot functions into a seperate file. This allows us to share the common code. Signed-off-by: Alistair Francis <alistair.francis@wdc.com> Reviewed-by: Bin Meng <bmeng.cn@gmail.com> Tested-by: Bin Meng <bmeng.cn@gmail.com> Signed-off-by: Palmer Dabbelt <palmer@sifive.com>