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>

arch/sandbox/config.mk

@@ -3,6 +3,7 @@
 
 PLATFORM_CPPFLAGS += -D__SANDBOX__ -U_FORTIFY_SOURCE
 PLATFORM_CPPFLAGS += -DCONFIG_ARCH_MAP_SYSMEM
+PLATFORM_CPPFLAGS += -fPIC
 PLATFORM_LIBS += -lrt
 
 # Define this to avoid linking with SDL, which requires SDL libraries