Add serial flow control C functions

https://forum.pjrc.com/threads/29446-Teensy-Hardware-Flow-Control-RTS-CTS?p=80425#post80425

teensy3/HardwareSerial.h

 void serial_format(uint32_t format);
 void serial_end(void);
 void serial_set_transmit_pin(uint8_t pin);
+int serial_set_rts(uint8_t pin);
+int serial_set_cts(uint8_t pin);
 void serial_putchar(uint32_t c);
 void serial_write(const void *buf, unsigned int count);
 void serial_flush(void);
 void serial2_format(uint32_t format);
 void serial2_end(void);
 void serial2_set_transmit_pin(uint8_t pin);
+int serial2_set_rts(uint8_t pin);
+int serial2_set_cts(uint8_t pin);
 void serial2_putchar(uint32_t c);
 void serial2_write(const void *buf, unsigned int count);
 void serial2_flush(void);
 void serial3_format(uint32_t format);
 void serial3_end(void);
 void serial3_set_transmit_pin(uint8_t pin);
+int serial3_set_rts(uint8_t pin);
+int serial3_set_cts(uint8_t pin);
 void serial3_putchar(uint32_t c);
 void serial3_write(const void *buf, unsigned int count);
 void serial3_flush(void);

teensy3/serial1.c

 	#endif
 }
 
+int serial_set_rts(uint8_t pin)
+{
+	if (!(SIM_SCGC4 & SIM_SCGC4_UART0)) return 0;
+	if (pin == 6) {
+		CORE_PIN6_CONFIG = PORT_PCR_MUX(3);
+	} else if (pin == 19) {
+		CORE_PIN19_CONFIG = PORT_PCR_MUX(3);
+	} else {
+		UART0_MODEM &= ~UART_MODEM_RXRTSE;
+		return 0;
+	}
+	UART0_MODEM |= UART_MODEM_RXRTSE;
+	return 1;
+}
+
+int serial_set_cts(uint8_t pin)
+{
+	if (!(SIM_SCGC4 & SIM_SCGC4_UART0)) return 0;
+	if (pin == 18) {
+		CORE_PIN18_CONFIG = PORT_PCR_MUX(3); // TODO: weak pullup or pulldown?
+	} else if (pin == 20) {
+		CORE_PIN20_CONFIG = PORT_PCR_MUX(3); // TODO: weak pullup or pulldown?
+	} else {
+		UART0_MODEM &= ~UART_MODEM_TXCTSE;
+		return 0;
+	}
+	UART0_MODEM |= UART_MODEM_TXCTSE;
+	return 1;
+}
+
 void serial_putchar(uint32_t c)
 {
 	uint32_t head, n;

teensy3/serial2.c

 	#endif
 }
 
+int serial2_set_rts(uint8_t pin)
+{
+	if (!(SIM_SCGC4 & SIM_SCGC4_UART1)) return 0;
+	if (pin == 22) {
+		CORE_PIN22_CONFIG = PORT_PCR_MUX(3);
+	} else {
+		UART1_MODEM &= ~UART_MODEM_RXRTSE;
+		return 0;
+	}
+	UART1_MODEM |= UART_MODEM_RXRTSE;
+	return 1;
+}
+
+int serial2_set_cts(uint8_t pin)
+{
+	if (!(SIM_SCGC4 & SIM_SCGC4_UART1)) return 0;
+	if (pin == 23) {
+		CORE_PIN23_CONFIG = PORT_PCR_MUX(3); // TODO: weak pullup or pulldown?
+	} else {
+		UART1_MODEM &= ~UART_MODEM_TXCTSE;
+		return 0;
+	}
+	UART1_MODEM |= UART_MODEM_TXCTSE;
+	return 1;
+}
+
 void serial2_putchar(uint32_t c)
 {
 	uint32_t head, n;

teensy3/serial3.c

 	#endif
 }
 
+int serial3_set_rts(uint8_t pin)
+{
+	if (!(SIM_SCGC4 & SIM_SCGC4_UART2)) return 0;
+	if (pin == 2) {
+		CORE_PIN2_CONFIG = PORT_PCR_MUX(3);
+	} else {
+		UART2_MODEM &= ~UART_MODEM_RXRTSE;
+		return 0;
+	}
+	UART2_MODEM |= UART_MODEM_RXRTSE;
+	return 1;
+}
+
+int serial3_set_cts(uint8_t pin)
+{
+	if (!(SIM_SCGC4 & SIM_SCGC4_UART2)) return 0;
+	if (pin == 14) {
+		CORE_PIN14_CONFIG = PORT_PCR_MUX(3); // TODO: weak pullup or pulldown?
+	} else {
+		UART2_MODEM &= ~UART_MODEM_TXCTSE;
+		return 0;
+	}
+	UART2_MODEM |= UART_MODEM_TXCTSE;
+	return 1;
+}
+
 void serial3_putchar(uint32_t c)
 {
 	uint32_t head, n;