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

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>