Add Alpine Linux support for Posix/Linux and FreeRTOS+TCP demos #1387

Description

The Posix/Linux simulator demo and the FreeRTOS+TCP Echo Posix demo currently fail to build and run on Alpine Linux due to musl‑specific header requirements and missing SLIRP development packages.

Problems observed

• Compilation errors on Alpine due to missing POSIX types (struct timeval, fd_set, select) because musl does not implicitly include <sys/time.h> or <sys/select.h>.
• Build failure when linking the TCP Echo demo because libslirp-dev is not installed by default on Alpine.
• Network initialization succeeds under SLIRP, but the demos do not document Alpine support or required dependencies.
• No setup script or guidance exists for Alpine users.

Why this matters

Alpine Linux is widely used for:

• lightweight development containers
• CI environments
• embedded Linux systems
• musl‑based toolchains

Supporting Alpine improves portability and ensures the Posix/Linux demos work across common Linux distributions.

Requested enhancements

• Add missing musl‑required includes to Posix/Linux demo sources.
• Add a setup script for Alpine (musl-dev, libslirp-dev).
• Update documentation to mention Alpine support and required packages.
• Ensure FreeRTOS+TCP Echo Posix demo builds cleanly under Alpine.
• Validate SLIRP networking behavior on musl systems.