sandbox: disable fortification

Since we provide all our own library calls, the fortification from
glibc just gets in our way (which some distros enable by default).

Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>

arch/sandbox/config.mk

@@ -17,5 +17,5 @@
 # Foundation, Inc., 59 Temple Place, Suite 330, Boston,
 # MA 02111-1307 USA
 
-PLATFORM_CPPFLAGS += -DCONFIG_SANDBOX -D__SANDBOX__
+PLATFORM_CPPFLAGS += -DCONFIG_SANDBOX -D__SANDBOX__ -U_FORTIFY_SOURCE
 PLATFORM_LIBS += -lrt