[sam] Fixed wrap-around bug in delay() (Mark Tillotson)

Fixes #1736

build/shared/revisions.txt

@@ -8,6 +8,9 @@ ARDUINO 1.5.6 BETA
 * TFT: warning messages in PImage class and strings inside examples now stored in flash to save RAM. 
 * Ethernet: added operator == for EthernetClient class (Norbert Truchsess)
 
+[core]
+* sam: Fixed wrap-around bug in delay() (Mark Tillotson)
+
 ARDUINO 1.5.5 BETA 2013.11.28
 
 NOTICE:

hardware/arduino/sam/cores/arduino/wiring.c

@@ -25,7 +25,7 @@ extern "C" {
 uint32_t millis( void )
 {
 // todo: ensure no interrupts
-	return GetTickCount() ;
+    return GetTickCount() ;
 }
 
 // Interrupt-compatible version of micros
@@ -74,9 +74,12 @@ uint32_t micros( void )
 
 void delay( uint32_t ms )
 {
-    uint32_t end = GetTickCount() + ms;
-    while (GetTickCount() < end)
-    	yield();
+    if (ms == 0)
+        return;
+    uint32_t start = GetTickCount();
+    do {
+        yield();
+    } while (GetTickCount() - start < ms);
 }
 
 #if defined ( __ICCARM__ ) /* IAR Ewarm 5.41+ */