Allow the trace output by mbed error to be conditional of NDEBUG.

This change avoid inclusion of printf and friends code in a binary when it
is compiled with the macro NDEBUG enabled (this macro is usually enabled
for production builds).

Unlike assert, the error function will still crash/halt the execution of the
application even if NDEBUG is enabled; the traces are just not outputed.

hal/common/mbed_error.c

@@ -24,9 +24,11 @@
 #endif
 
 WEAK void error(const char* format, ...) {
+#ifndef NDEBUG
     va_list arg;
     va_start(arg, format);
     mbed_error_vfprintf(format, arg);
     va_end(arg);
+#endif
     exit(1);
 }