py/mpconfig,ports: Define new HEX_FMT formatting macro.

Signed-off-by: Jeff Epler <jepler@gmail.com>

ports/stm32/mpconfigport_nanbox.h

@@ -36,6 +36,7 @@
 // Types needed for nan-boxing
 #define UINT_FMT "%llu"
 #define INT_FMT "%lld"
+#define HEX_FMT "%llx"
 typedef int64_t mp_int_t;
 typedef uint64_t mp_uint_t;