Fix all warnings

Fixes #581

Author: facchinm

Diff for: cores/arduino/Arduino.h

@@ -31,6 +31,7 @@ extern "C"{
 #endif // __cplusplus
 
 // Include Atmel headers
+#undef LITTLE_ENDIAN
 #include <samd.h>
 
 #define clockCyclesPerMicrosecond() ( SystemCoreClock / 1000000L )

Diff for: cores/arduino/USB/USBCore.cpp

@@ -230,7 +230,6 @@ bool USBDeviceClass::sendDescriptor(USBSetup &setup)
 		else if (setup.wValueL == ISERIAL) {
 			char name[ISERIAL_MAX_LEN];
 			memset(name, 0, sizeof(name));
-			uint8_t idx = 0;
 #ifdef PLUGGABLE_USB_ENABLED
 			PluggableUSB().getShortName(name);
 			return sendStringDescriptor((uint8_t*)name, setup.wLength);

Diff for: cores/arduino/USB/samd21_host.c

@@ -25,7 +25,6 @@
 #include "variant.h"
 #include "USB_host.h"
 #include "samd21_host.h"
-#include "sam.h"
 #include "wiring_private.h"
 
 #define HOST_DEFINED

Diff for: libraries/SPI/SPI.h

@@ -28,7 +28,8 @@
 //   - endTransaction()
 //   - usingInterrupt()
 //   - SPISetting(clock, bitOrder, dataMode)
-#define SPI_HAS_TRANSACTION 1
+// #define SPI_HAS_TRANSACTION 1
+// Every core implementing arduino API has SPI transaction
 
 // SPI_HAS_NOTUSINGINTERRUPT means that SPI has notUsingInterrupt() method
 #define SPI_HAS_NOTUSINGINTERRUPT 1