The same H/W UART may be shared by multiple serial_t objects. This fix tries to avoid
re-configuring the same H/W UART in serial_init() when there are multiple serial_t
objects constructed. To re-configure UART, call serial_baud() and serial_format()
explicitly. This can avoid confusion when e.g. a newly constructed serial_t object
changes baudrate unexpectedly in serial_init().
Serial implementation uses different vector handlers for sync/async calls respectively. The issue can be reproduced with the following flow:
1. Register sync mode callback with Serial.attach().
2. Sync call with Serial.putc()/getc().
3. Change to async call with Serial.write()/read().
4. Change back to sync call with Serial.putc()/getc().
Now, vector handller is still for async mode, not for sync mode.
To fix it:
1. Introduce internal function serial_enable_interrupt() for both sync/async vector handler enable/disable.
Original HAL function serial_irq_set() is reduced to call it for sync mode vector handler enable/disable.
2. Introduce internal function serial_rollback_interrupt() to roll back sync mode vector handler at end of async transfer.
ccli8