sandbox: Build with -fPIC

Sandbox is not a real bootloader and it does require
a position independent code to be supported.

Thus, build it with -fPIC explicitly.

Fixes: 16940f720f9b ("Makefile: Don't generate position independent code")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reported-by: Simon Glass <sjg@chromium.org>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>