disable security warning flags when possible

Some toolchains enable security warning flags by default, but these don't
really make sense in the u-boot world.  Such as forcing changes like:
	-printf(foo);
	+printf("%s", foo);

So disable the flags when the compiler supports them.  Linux has already
merged a similar change in their build system.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/config.mk b/config.mk
index ed44d03..e2b440d 100644
--- a/config.mk
+++ b/config.mk
@@ -211,6 +211,10 @@
 endif
 
 CFLAGS += $(call cc-option,-fno-stack-protector)
+# Some toolchains enable security related warning flags by default,
+# but they don't make much sense in the u-boot world, so disable them.
+CFLAGS += $(call cc-option,-Wno-format-nonliteral)
+CFLAGS += $(call cc-option,-Wno-format-security)
 
 # $(CPPFLAGS) sets -g, which causes gcc to pass a suitable -g<format>
 # option to the assembler.