sandbox: Don't include the main loop in SPL

SPL does not have a command interface so we should not include the main loop
code.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/sandbox/cpu/start.c

@@ -73,6 +73,7 @@ static int sandbox_cmdline_cb_help(struct sandbox_state *state, const char *arg)
 }
 SANDBOX_CMDLINE_OPT_SHORT(help, 'h', 0, "Display help");
 
+#ifndef CONFIG_SPL_BUILD
 int sandbox_main_loop_init(void)
 {
 	struct sandbox_state *state = state_get_current();
@@ -97,6 +98,7 @@ int sandbox_main_loop_init(void)
 
 	return 0;
 }
+#endif
 
 static int sandbox_cmdline_cb_boot(struct sandbox_state *state,
 				      const char *arg)