Fix STM non-F4 builds

Author: tannewt

ports/stm/supervisor/serial.c

@@ -28,14 +28,16 @@
 #include "py/mphal.h"
 #include <string.h>
 #include "supervisor/serial.h"
+#if CPY_STM32F4
 #include "stm32f4xx_hal.h"
 #include "stm32f4/gpio.h"
-
 // TODO: Switch this to using DEBUG_UART.
 
 UART_HandleTypeDef huart2;
+#endif
 
 void port_serial_init(void) {
+    #if CPY_STM32F4
     huart2.Instance = USART2;
     huart2.Init.BaudRate = 115200;
     huart2.Init.WordLength = UART_WORDLENGTH_8B;
@@ -47,25 +49,33 @@ void port_serial_init(void) {
     if (HAL_UART_Init(&huart2) == HAL_OK) {
         stm32f4_peripherals_status_led(1,1);
     }
+    #endif
 }
 
 bool port_serial_connected(void) {
     return true;
 }
 
 char port_serial_read(void) {
+    #if CPY_STM32F4
     uint8_t data;
     HAL_UART_Receive(&huart2, &data, 1,500);
     return data;
+    #else
+    return -1;
+    #endif
 }
 
 bool port_serial_bytes_available(void) {
+    #if CPY_STM32F4
     return __HAL_UART_GET_FLAG(&huart2, UART_FLAG_RXNE);
+    #else
+    return false;
+    #endif
 }
 
 void port_serial_write_substring(const char *text, uint32_t len) {
-    if (len == 0) {
-        return;
-    }
+    #if CPY_STM32F4
     HAL_UART_Transmit(&huart2, (uint8_t *)text, len, 5000);
+    #endif
 }