malloc_simple: Add Kconfig option for using only malloc_simple in the SPL

common/dlmalloc.c is quite big, both in .text and .data usage, therefor
on some boards the SPL is build to use only malloc_simple.c and not the
dlmalloc.c code. This is done in various include/configs/foo.h with the
following construct:

 #ifdef CONFIG_SPL_BUILD
 #define CONFIG_SYS_MALLOC_SIMPLE
 #endif

This commit introduces a SPL_MALLOC_SIMPLE Kconfig bool which allows
selecting this functionality through Kconfig instead.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/common/malloc_simple.c b/common/malloc_simple.c
index c745863..9bf1fed 100644
--- a/common/malloc_simple.c
+++ b/common/malloc_simple.c
@@ -40,7 +40,7 @@
 	return ptr;
 }
 
-#ifdef CONFIG_SYS_MALLOC_SIMPLE
+#if CONFIG_IS_ENABLED(SYS_MALLOC_SIMPLE)
 void *calloc(size_t nmemb, size_t elem_size)
 {
 	size_t size = nmemb * elem_size;