x86: edison: Move config SYS_MALLOC_LEN to Kconfig

This patch moves the the config SYS_MALLOC_LEN to Kconfig
as it is already done for zynq arch in commit 01aa5b8f
("Kconfig: Move config SYS_MALLOC_LEN to Kconfig for zynq").

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>