riscv: Modify generic codes to support RISC-V

Support common commands bdinfo and image format,
also modify common generic flow for RISC-V.

Signed-off-by: Rick Chen <rick@andestech.com>
Signed-off-by: Rick Chen <rickchen36@gmail.com>
Signed-off-by: Greentime Hu <green.hu@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>