add calculate CRC function

inc/rs485.h

@@ -12,6 +12,7 @@
 #include <string.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <sys/types.h>
 
 #define RS485_RCU            RCU_USART0
 #define RS485_GPIO_RCU       RCU_GPIOA

src/rs485.c

@@ -53,3 +53,18 @@ void process_command(char *cmd) {
         printf("Invalid Command!\r\n");
     }
 }
+
+uint8_t calculate_crc(uint8_t package_header[3], uint8_t package_data[], size_t data_length) {
+    uint8_t crc = 0;
+
+    /* Calculate CRC for header */
+    for (int i = 1; i < 3; i++) {
+        crc += package_header[i];
+    }
+
+    for (size_t i = 0; i < data_length; i++) {
+        crc += package_data[i];
+    }
+
+    return (uint8_t)(crc & 0xFF);
+}