dlmalloc: do memset in malloc init as new default config

This commit introduces new config: CONFIG_SYS_MALLOC_CLEAR_ON_INIT.

This config is an expert option and is enabled by default.

The all amount of memory reserved for the malloc, is by default set
to zero in mem_malloc_init(). When the malloc reserved memory exceeds
few MiB, then the boot process can slow down.

So disabling this config, is an expert option to reduce the boot time,
and can be disabled by Kconfig.

Note:
After disable this option, only calloc() will return the pointer
to the zeroed memory area. Previously, without this option,
the memory pointed to untouched malloc memory region, was filled
with zeros. So it means, that code with malloc() calls should
be reexamined.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

common/dlmalloc.c

@@ -1535,9 +1535,9 @@ void mem_malloc_init(ulong start, ulong size)
 
 	debug("using memory %#lx-%#lx for malloc()\n", mem_malloc_start,
 	      mem_malloc_end);
-
-	memset((void *)mem_malloc_start, 0, size);
-
+#ifdef CONFIG_SYS_MALLOC_CLEAR_ON_INIT
+	memset((void *)mem_malloc_start, 0x0, size);
+#endif
 	malloc_bin_reloc();
 }
 
@@ -2948,9 +2948,11 @@ Void_t* cALLOc(n, elem_size) size_t n; size_t elem_size;
 
 
   /* check if expand_top called, in which case don't need to clear */
+#ifdef CONFIG_SYS_MALLOC_CLEAR_ON_INIT
 #if MORECORE_CLEARS
   mchunkptr oldtop = top;
   INTERNAL_SIZE_T oldtopsize = chunksize(top);
+#endif
 #endif
   Void_t* mem = mALLOc (sz);
 
@@ -2977,12 +2979,14 @@ Void_t* cALLOc(n, elem_size) size_t n; size_t elem_size;
 
     csz = chunksize(p);
 
+#ifdef CONFIG_SYS_MALLOC_CLEAR_ON_INIT
 #if MORECORE_CLEARS
     if (p == oldtop && csz > oldtopsize)
     {
       /* clear only the bytes from non-freshly-sbrked memory */
       csz = oldtopsize;
     }
+#endif
 #endif
 
     MALLOC_ZERO(mem, csz - SIZE_SZ);