core: Improve Serial implements

[soburi]

• Refactored ZephyrSerial's send/receive logic and protected the RX/TX ring buffers with semaphores.
• Modified write() to write the entire payload by calling yield() if the data exceeds buffer capacity.
• Added support for availableForWrite(), flush(), and the Print::write overload to align with the Arduino Core implementation.
• UART re-initialization logic move to _reinit_if_needed().
• Added a mechanism to shift the standard UART to Serial1 (or higher) if Serial is occupied by cdc_acm_serial or arduino_router_serial.
• Added macros (such as ARDUINO_CONSOLE_SERIAL and ARDUINO_HARDWARE_SERIAL) to reference Serial objects based on their specific use cases.

[Copilot]

Pull request overview

This PR refactors the Arduino Zephyr serial implementation to better align with Arduino Core semantics (buffering, Print overloads, availableForWrite(), and flush()), and adds Devicetree-driven macros to select the appropriate Serial* object for console/hardware/USB/router use cases.

Changes:

• Refactors TX/RX buffering and introduces semaphore-protected access patterns, plus UART re-init via _reinit_if_needed().
• Updates write() behavior to push the full payload (yielding when buffers are full), and adds availableForWrite()/flush()/Print::write overload visibility.
• Adds macro infrastructure to map/alias Serial objects based on Devicetree (console serial, hardware serial, USB CDC ACM serial, and router serial).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
cores/arduino/zephyrSerial.h	Adds new serial selection/indexing macros and updates the ZephyrSerial interface (flush(), availableForWrite(), Print::write overloads).
cores/arduino/zephyrSerial.cpp	Refactors IRQ handling and TX/RX operations, implements flush() and availableForWrite(), and updates serial object instantiation/event hooks.

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