HAL_ChibiOS: added vsnprintf wrapper

this allows us to use a common snprintf implementation

libraries/AP_HAL_ChibiOS/hwdef/common/stdio.h

@@ -26,6 +26,7 @@ extern "C" {
 
 int vsnprintf(char *str, size_t size, const char *fmt, va_list ap);
 int __wrap_snprintf(char *str, size_t size, const char *fmt, ...);
+int __wrap_vsnprintf(char *str, size_t size, const char *fmt, va_list ap);
 int snprintf(char *str, size_t size, const char *fmt, ...); //undefined, only used as a placeholder, its replaced by wrap method at link time
 int vasprintf(char **strp, const char *fmt, va_list ap);
 int asprintf(char **strp, const char *fmt, ...);

libraries/AP_HAL_ChibiOS/stdio.cpp

@@ -44,6 +44,11 @@ int __wrap_snprintf(char *str, size_t size, const char *fmt, ...)
    return done;
 }
 
+int __wrap_vsnprintf(char *str, size_t size, const char *fmt, va_list ap)
+{
+    return hal.util->vsnprintf(str, size, fmt, ap);
+}
+
 int vasprintf(char **strp, const char *fmt, va_list ap)
 {
     int len = vsnprintf(NULL, 0, fmt, ap);