Use of Serial.end() gives linking error

[sticilface]

If serial.end() is called the following error is produced.
Appeared in commit e255f25

.pioenvs/nodemcu2/libFrameworkArduino.a(uart.o):(.text.uart_uninit+0x0): undefined reference to `os_free'
.pioenvs/nodemcu2/libFrameworkArduino.a(uart.o): In function `uart_uninit':
uart.c:(.text.uart_uninit+0x37): undefined reference to `os_free'
collect2: error: ld returned 1 exit status

[igrr]

Oh, sorry, that was my previous pull request. Will fix.