sandbox: Disable built-in malloc

We prefer to U-Boot's malloc but for now it is easier to use the C library's
version.

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

common/Makefile

@@ -29,7 +29,9 @@ LIB	= $(obj)libcommon.o
 ifndef CONFIG_SPL_BUILD
 COBJS-y += main.o
 COBJS-y += command.o
+ifndef CONFIG_SANDBOX
 COBJS-y += dlmalloc.o
+endif
 COBJS-y += exports.o
 COBJS-$(CONFIG_SYS_HUSH_PARSER) += hush.o
 COBJS-y += image.o