Review millisDiff

Found out here, Libelium/waspmoteapi#15 that
this could be much more simple.

If I have not removed the function enterily it's because it would take
more program memory, something I want to avoid now (we run out of
program memory). That's why the function is declared noinline.

libraries/Coroutines/Coroutines.cpp

@@ -253,16 +253,15 @@ char* strnjoin_F(char* dst, size_t size, const __FlashStringHelper* delimiter, c
 /*
  * Functions to calculate the distance between two calls to millis(), taking
  * into account overflow.
+ *
+ * XXX We could just remove this function and use millis() - t0 everywhere, but
+ * it would use more program memory, which we want to avoid, at least for now.
+ * That's why we declare the function noinline in the header file.
  */
 
-uint32_t Loop::millisDiff(uint32_t t0, uint32_t t1)
-{
-  return (t1 >= t0) ? t1 - t0: (ULONG_MAX - t0) + t1;
-}
-
 uint32_t Loop::millisDiff(uint32_t t0)
 {
-    return millisDiff(t0, millis());
+  return millis() - t0;
 }
 
 /*

libraries/Coroutines/Coroutines.h

@@ -157,8 +157,7 @@ class Loop
     // millisDiff is to calculate the difference between two calls to millis(),
     // but taking into account overflow. This is only needed if the device runs
     // without sleep for longer than 49 days.
-    uint32_t millisDiff(uint32_t t0, uint32_t t1);
-    uint32_t millisDiff(uint32_t t0);
+    uint32_t millisDiff(uint32_t t0) __attribute__((noinline));
 
     // Printing to USB done right. The print functions takes a formatted
     // string, either a regular string (char*) or one stored in the program