risc-v/virt: enable gradeful shutdown in kernel build

This enables graceful shutdown via `poweroff` command in `knsh64`
and `knsh32` configs.

Signed-off-by: Yanfeng Liu <yfliu2008@qq.com>

boards/risc-v/qemu-rv/rv-virt/configs/knsh32/defconfig

@@ -42,6 +42,7 @@ CONFIG_ARCH_USE_MMU=y
 CONFIG_ARCH_USE_MPU=y
 CONFIG_ARCH_USE_S_MODE=y
 CONFIG_BINFMT_ELF_EXECUTABLE=y
+CONFIG_BOARDCTL_POWEROFF=y
 CONFIG_BOARD_LOOPSPERMSEC=6366
 CONFIG_BUILD_KERNEL=y
 CONFIG_DEBUG_ASSERTIONS=y

boards/risc-v/qemu-rv/rv-virt/configs/knsh64/defconfig

@@ -42,6 +42,7 @@ CONFIG_ARCH_USE_MMU=y
 CONFIG_ARCH_USE_MPU=y
 CONFIG_ARCH_USE_S_MODE=y
 CONFIG_BINFMT_ELF_EXECUTABLE=y
+CONFIG_BOARDCTL_POWEROFF=y
 CONFIG_BOARD_LOOPSPERMSEC=6366
 CONFIG_BUILD_KERNEL=y
 CONFIG_DEBUG_ASSERTIONS=y
@@ -62,7 +63,6 @@ CONFIG_INIT_MOUNT_FSTYPE="hostfs"
 CONFIG_INIT_MOUNT_SOURCE=""
 CONFIG_INIT_MOUNT_TARGET="/system"
 CONFIG_INIT_STACKSIZE=3072
-CONFIG_INTELHEX_BINARY=y
 CONFIG_LIBC_ENVPATH=y
 CONFIG_LIBC_EXECFUNCS=y
 CONFIG_LIBC_PERROR_STDOUT=y

boards/risc-v/qemu-rv/rv-virt/src/qemu_rv_appinit.c

@@ -36,6 +36,7 @@
 
 #include <sys/mount.h>
 
+#include "riscv_internal.h"
 #include "romfs.h"
 
 /****************************************************************************
@@ -177,3 +178,17 @@ void board_late_initialize(void)
 
 #endif
 }
+
+#ifdef CONFIG_BOARDCTL_POWEROFF
+int board_power_off(int status)
+{
+#ifdef CONFIG_BUILD_KERNEL
+  riscv_sbi_system_reset(SBI_SRST_TYPE_SHUTDOWN, SBI_SRST_REASON_NONE);
+#else
+  #warning "to be done"
+#endif
+
+  UNUSED(status);
+  return 0;
+}
+#endif