x86: Drop CFG_SYS_STACK_SIZE

This is only used in one file and the value is the same for both boards
which define it. Use the fixed value of 32KB and drop the CFG. This will
allow removal of the config.h files.

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