qemu-riscv64_smode: fix extlinux (define preboot)

Commit 37304aaf ("Convert CONFIG_USE_PREBOOT and CONFIG_PREBOOT to
Kconfig") removed preboot commands in RISC-V targets and broke
extlinux support as reported by Fu Wei <wefu@redhat.com>.

The patch finishes migration of CONFIG_USE_PREBOOT and CONFIG_REBOOT
to Kconfig.

Fixes: 37304aaf ("Convert CONFIG_USE_PREBOOT and CONFIG_PREBOOT to Kconfig")
Reported-By: Fu Wei <wefu@redhat.com>
Signed-off-by: David Abdurachmanov <david.abdurachmanov@sifive.com>
Signed-off-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>