sandbox: Add an implementation for cleanup_before_linux_select()

Support this function so we can use Chrome OS verified boot with sandbox.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/cpu/cpu.c b/arch/sandbox/cpu/cpu.c
index 02c4cd3..e6ddb17 100644
--- a/arch/sandbox/cpu/cpu.c
+++ b/arch/sandbox/cpu/cpu.c
@@ -52,6 +52,11 @@
 	return 0;
 }
 
+int cleanup_before_linux_select(int flags)
+{
+	return 0;
+}
+
 void *map_physmem(phys_addr_t paddr, unsigned long len, unsigned long flags)
 {
 #ifdef CONFIG_PCI