M2354: Add serial_tx_empty(...)
M2354: Add serial_tx_empty(...)

Enable serial sync with the below PR:
https://github.com/ARMmbed/mbed-os/pull/14600

mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed