Extern C blocks (#1352) (#4044)

* Add extern C guard blocks to SDK header files #1352

* fixed some extern C blocks in core and libraries

tools/sdk/include/at_custom.h

@@ -27,6 +27,10 @@
 
 #include "c_types.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 typedef struct
 {
   char *at_cmdName;
@@ -169,4 +173,8 @@ bool at_fake_uart_enable(bool enable,at_fake_uart_tx_func_type at_fake_uart_tx_f
   */
 bool at_set_escape_character(uint8 ch);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif