Wrap PySerial inside busio.UART for Linux platforms - #1106

Summary

Fixes #332.

Instead of raising RuntimeError on embedded Linux, busio.UART now wraps PySerial via a new generic_linux UART backend. This lets CircuitPython UART code run unchanged on Linux / SBC boards (Raspberry Pi, BeagleBone, etc.).

Changes

New file: src/adafruit_blinka/microcontroller/generic_linux/uart.py

• PySerial-backed UART with full CircuitPython API
• Resolves port IDs to /dev/ device paths automatically (/dev/serial0, /dev/ttyS0, /dev/ttyAMA0, etc.)
• Supports string device paths for direct use
• Methods: read, readinto, readline, write, deinit, reset_input_buffer
• Properties: baudrate, in_waiting, timeout

Updated: src/busio.py

• UART.__init__ now imports generic_linux.uart when detector.board.any_embedded_linux is true (instead of raising RuntimeError)
• Follows the same pattern used by busio.I2C and busio.SPI for Linux support
• Added missing CircuitPython API members:
  • timeout property (getter/setter)
  • __enter__() / __exit__() context manager support
  • reset_input_buffer() method
  • baudrate as a proper property (getter/setter) instead of a plain attribute
• Fixed timeout default from 1000 (stale milliseconds value) to 1 (seconds), matching CircuitPython 4.0+ convention
• Improved deinit() to call the backend's deinit() when available

Usage

import board
import busio

# Works on Linux now — no more RuntimeError!
uart = busio.UART(board.TX, board.RX, baudrate=115200)
data = uart.read(32)

# Context manager support
with busio.UART(board.TX, board.RX, baudrate=9600) as uart:
    uart.write(b'hello')

Notes

• PySerial (pyserial) must be installed but is not added to install_requires — it's already commonly available on Linux SBC setups and is used elsewhere in the project (e.g. RP2040 U2IF)
• The port ID mapping uses uartPorts from the board's pin definitions (same as I2C uses i2cPorts)