adb: add serial transport support for microADB

[fangpeina]

Add a serial (UART) transport backend using uv_tty_t on a bidirectional serial fd configured in raw mode via termios. Reuses existing frame assembly and backpressure mechanisms. Designed for resource-constrained environments where neither network nor USB is available.

Verified on Linux Host (PTY loopback)

# 1. Build (standalone CMake)
mkdir -p build && cd build
cmake .. -DADBD_SERIAL_SERVER=ON \
         -DADBD_SERIAL_PORT="/tmp/ttyV0" \
         -DADBD_SERIAL_BAUDRATE=921600
cmake --build . -j

# 2. Create virtual serial pair
socat PTY,raw,echo=0,link=/tmp/ttyV0 PTY,raw,echo=0,link=/tmp/ttyV1 &

# 3. Bridge serial to TCP (for host adb client)
socat TCP-LISTEN:5558,reuseaddr,nodelay FILE:/tmp/ttyV1,raw &

# 4. Start adbd
./adbd

# 5. Connect from PC
adb connect localhost:5558
adb devices
adb -s localhost:5558 shell pwd
adb -s localhost:5558 shell uname -a
adb shell