spl: Use SYS_MALLOC_F instead of SYS_MALLOC_F_LEN

Use the new SPL/TPL/VPL_SYS_MALLOC_F symbols to determine whether the
malloc pool exists.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Sean Anderson <sean.anderson@seco.com>
diff --git a/arch/sandbox/cpu/start.c b/arch/sandbox/cpu/start.c
index 1026898..bbd9e77 100644
--- a/arch/sandbox/cpu/start.c
+++ b/arch/sandbox/cpu/start.c
@@ -529,7 +529,7 @@
 			goto err;
 	}
 
-#if CONFIG_VAL(SYS_MALLOC_F_LEN)
+#if CONFIG_IS_ENABLED(SYS_MALLOC_F)
 	gd->malloc_base = CFG_MALLOC_F_ADDR;
 #endif
 #if CONFIG_IS_ENABLED(LOG)