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>