Add BIOS and tiny kernel roadmap phases

Tutorial/phase-20-linux/phase-20.md

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+# Phase 20 - Linux
+
+## Context
+
+This is the summit: boot a minimal Linux kernel with a BusyBox initramfs to a shell over
+UART on the CPU and SoC you built.
+
+## Goals
+
+- Build a kernel matching the implemented ISA and platform.
+- Provide DTB and initramfs.
+- Reach an interactive shell or a controlled init process.
+
+## New Concepts
+
+- Kernel config: set of build-time options selecting architecture and drivers.
+- Initramfs: initial root filesystem bundled or loaded with the kernel.
+- BusyBox: compact Unix userland used in embedded systems.
+- Early console: minimal logging path before normal console drivers initialize.
+- Root filesystem: filesystem mounted as `/` by Linux.
+
+## How To Think About It
+
+Linux bring-up is system debugging under poor visibility. Work from known-good layers:
+CPU tests, DRAM test, boot firmware, DTB validation, early console, then userspace.
+
+## Learning Tasks
+
+- Understand the kernel image format and load addresses for RV32.
+- Build the smallest kernel config that matches your hardware.
+- Trace boot log milestones and map them to hardware dependencies.
+
+## Pitfalls
+
+- Debugging Linux before compliance tests and DRAM tests are clean.
+- Enabling drivers for devices you do not actually implement.
+- Losing console output because UART binding or clock frequency is wrong.
+
+## Tooling And Testing
+
+- Use earlycon/earlyprintk-style mechanisms where applicable.
+- Keep kernel, DTB, firmware, and bitstream versions tied together.
+- Save full boot logs and compare against previous attempts.
+
+## References
+
+- Linux RISC-V documentation: https://docs.kernel.org/arch/riscv/
+- Buildroot: https://buildroot.org/
+- BusyBox: https://busybox.net/
+