Add crc16.h for AVR libc compatibility

teensy3/util/crc16.h

+#ifndef _UTIL_CRC16_H_
+#define _UTIL_CRC16_H_
+
+#include <stdint.h>
+
+static inline uint16_t _crc16_update(uint16_t crc, uint8_t data) __attribute__((always_inline, unused));
+static inline uint16_t _crc16_update(uint16_t crc, uint8_t data)
+{
+	unsigned int i;
+
+	crc ^= data;
+	for (i = 0; i < 8; ++i) {
+		if (crc & 1) {
+			crc = (crc >> 1) ^ 0xA001;
+		} else {
+			crc = (crc >> 1);
+		}
+	}
+	return crc;
+}
+
+static inline uint16_t _crc_xmodem_update(uint16_t crc, uint8_t data) __attribute__((always_inline, unused));
+static inline uint16_t _crc_xmodem_update(uint16_t crc, uint8_t data)
+{
+	unsigned int i;
+
+	crc = crc ^ ((uint16_t)data << 8);
+	for (i=0; i<8; i++) {
+		if (crc & 0x8000) {
+			crc = (crc << 1) ^ 0x1021;
+		} else {
+			crc <<= 1;
+		}
+	}
+	return crc;
+}
+
+static inline uint16_t _crc_ccitt_update (uint16_t crc, uint8_t data) __attribute__((always_inline, unused));
+static inline uint16_t _crc_ccitt_update (uint16_t crc, uint8_t data)
+{
+	data ^= (crc & 255);
+	data ^= data << 4;
+
+	return ((((uint16_t)data << 8) | (crc >> 8)) ^ (uint8_t)(data >> 4) 
+		^ ((uint16_t)data << 3));
+}
+
+static inline uint8_t _crc_ibutton_update(uint8_t crc, uint8_t data) __attribute__((always_inline, unused));
+static inline uint8_t _crc_ibutton_update(uint8_t crc, uint8_t data)
+{
+	unsigned int i;
+
+	crc = crc ^ data;
+	for (i = 0; i < 8; i++) {
+		if (crc & 0x01) {
+			crc = (crc >> 1) ^ 0x8C;
+		} else {
+			crc >>= 1;
+		}
+	}
+	return crc;
+}
+
+#endif
+