bugfix(tools): idf.py monitor now reads correctly on Linux

A missing flush in the serial reader implementation for
Linux target was causing input to idf.py monitor to not
be forwarded to the application. This is fixed now.

tools/idf_monitor.py

@@ -267,6 +267,7 @@ class LinuxMonitor(Monitor):
 
     def serial_write(self, *args, **kwargs):  # type: ignore
         self.serial.stdin.write(*args, **kwargs)
+        self.serial.stdin.flush()
 
     def check_gdb_stub_and_run(self, line: bytes) -> None:
         return  # fake function for linux target