Fixed some bugs in the Serial implementations

[mikljohansson]

tl;dr I fixed return value handling of UARTClass::write and Print::write

• UARTClass::write was always return 0 as the number of bytes written. It should return 1, which is the number of bytes written to the UART
• Print::write(buf, len) method wasn't considering the return values from the write(c) method
• uarths_rec_callback() was using deprecated Kendryte SDK function uarths_getc()
• HardwareSerial::availableForWrite() method wasn't exposed in the interface