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/include/malloc.h b/include/malloc.h
index f4da9e6..f20e4d3 100644
--- a/include/malloc.h
+++ b/include/malloc.h
@@ -872,7 +872,7 @@
 
 #else
 
-#ifdef CONFIG_SYS_MALLOC_SIMPLE
+#if CONFIG_IS_ENABLED(SYS_MALLOC_SIMPLE)
 #define malloc malloc_simple
 #define realloc realloc_simple
 #define memalign memalign_simple