When 'make qemu-riscv64_defconfig', there is a build warning:

  board/emulation/qemu-riscv/Kconfig:24:
  warning: config symbol defined without type

Fix it by specifying the config symbol type to 'hex'.
Signed-off-by: NBin Meng <bmeng.cn@gmail.com>
Reviewed-by: NRick Chen <rick@andestech.com>
Reviewed-by: NLukas Auer <lukas.auer@aisec.fraunhofer.de>