commit for customized Makefile and build script. && Implemented a User_Main function call demo as a bridge to allow program execution to transition from main.c to C++ environment.

User/Main/Src/User_Main.cpp

@@ -0,0 +1,12 @@
+extern "C"
+{
+#include "usart.h"
+}
+
+// 自定义函数入口
+extern "C" void User_Main(void)
+{
+    HAL_UART_Transmit(&huart2, (const uint8_t *)"Hello world from C++ !!\r\n", 25, 1000);
+
+    return;
+}