Add missing AVR eeprom functions

teensy3/avr_functions.h

 void eeprom_write_word(uint16_t *addr, uint16_t value);
 void eeprom_write_dword(uint32_t *addr, uint32_t value);
 void eeprom_write_block(const void *buf, void *addr, uint32_t len);
+int eeprom_is_ready(void);
+#define eeprom_busy_wait() do {} while (!eeprom_is_ready())
 
 static inline float eeprom_read_float(const float *addr) __attribute__((pure, always_inline, unused));
 static inline float eeprom_read_float(const float *addr)

teensy3/eeprom.c

 	}
 }
 
+int eeprom_is_ready(void)
+{
+	return (FTFL_FCNFG & FTFL_FCNFG_EEERDY) ? 1 : 0;
+}
+
 static void flexram_wait(void)
 {
 	while (!(FTFL_FCNFG & FTFL_FCNFG_EEERDY)) {