board_init_f_mem(): Don't create an unused early malloc() area

Change the #ifdef so that the early malloc() area is not set up in SPL if
CONFIG_SYS_SPL_MALLOC_START is defined. In that case it would never actually
be used, and just chews up stack space.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Masahiro Yamada <yamada.masahiro@socionext.com>