AP_Common: add scanf format macro

Add a macro to annotate functions that act like scanf.  Calling the
printf format macro as FORMAT was bad as can be seen now. Later we need
to rename it to FMT_PRINTF.

libraries/AP_Common/AP_Common.h

@@ -44,6 +44,7 @@
 #define NOINLINE __attribute__((noinline))
 
 #define FORMAT(a,b) __attribute__((format(printf, a, b)))
+#define FMT_SCANF(a,b) __attribute__((format(scanf, a, b)))
 
 // Make some dire warnings into errors
 //