Add RT-Thread port and package integration

[wdfk-prog]

Pull request

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• bug
• feature

Describe the pull request

Add RT-Thread integration support for the eRPC C/C++ runtime.

This pull request introduces:

• Native RT-Thread threading backend support.

• RT-Thread heap allocation port using rt_malloc() and rt_free().

• RT-Thread assertion mapping through RT_ASSERT().

• RT-Thread UART framed transport based on the RT-Thread device API.

• RT-Thread UART setup helper:

  • erpc_transport_rtthread_uart_init()
  • erpc_transport_rtthread_uart_deinit()

• RT-Thread-specific erpc_config.h for package builds.

• RT-Thread Kconfig options for selecting runtime roles, message buffer factory, feature macros, endianness settings, and transport components.

• RT-Thread SConscript build integration for selecting sources according to menuconfig.

• Documentation updates in the top-level README and rtthread/README.md.

The integration is intended for MCU targets running RT-Thread, especially projects that use generated eRPC client/server stubs with UART as the transport.

To Reproduce

Without this change, eRPC does not provide a native RT-Thread package integration path.

Typical RT-Thread package usage requires:

1. Adding an eRPC Kconfig entry to the RT-Thread BSP/package configuration.
2. Running:

scons --menuconfig

3. Enabling eRPC runtime components.
4. Building the selected eRPC C/C++ runtime sources through SCons.
5. Initializing a transport, for example:

erpc_transport_t transport = erpc_transport_rtthread_uart_init("uart2", 115200);

Before this PR, the RT-Thread-specific backend, UART transport, configuration header, Kconfig options, and SCons source selection were not available.

Expected behavior

RT-Thread users can enable eRPC through menuconfig, select the required client/server/runtime components, and build the eRPC runtime with SCons.

For UART transport, users can initialize an RT-Thread UART device by name and baud rate, then use it as an eRPC framed transport for client or server communication.

The RT-Thread backend should provide:

• Thread creation and startup through RT-Thread APIs.
• Mutex and semaphore support through RT-Thread IPC primitives.
• Sleep and timeout conversion from eRPC microsecond values to RT-Thread ticks.
• Dynamic memory allocation through the RT-Thread heap.
• UART send/receive through the RT-Thread device API.

Screenshots

N/A

Desktop (please complete the following information):

• OS: RT-Thread target environment / host build environment as applicable
• eRPC Version: current upstream branch

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Additional context

The RT-Thread UART transport owns the RT-Thread rx_indicate callback while active. Applications should not install another receive callback on the same UART device during the transport lifetime.

The UART transport uses a fixed registry for mapping UART devices to active transport instances. The registry size is configurable through the RT-Thread package option.

This PR does not build the host-side erpcgen tool for MCU targets. It focuses on the eRPC runtime used by generated client/server stubs in RT-Thread projects.