Adding printf format warning flags to makefiles

Add printf format warning flags that are supported by GCC 3.0 or later
to the makefiles

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/library/Makefile b/library/Makefile
index 903dc0d..6bb9c17 100644
--- a/library/Makefile
+++ b/library/Makefile
@@ -2,7 +2,7 @@
 # Also see "include/mbedtls/config.h"
 
 CFLAGS	?= -O2
-WARNING_CFLAGS ?=  -Wall -Wextra
+WARNING_CFLAGS ?=  -Wall -Wextra -Wformat=2 -Wno-format-nonliteral
 LDFLAGS ?=
 
 # Include ../include for public headers and . for private headers.