Provide a minimal firmware validator

[fhunleth]

The docs contain everything to enable it and will work on all Nerves systems. For firmware validation, nerves_system_qemu_aarch64 is the only one that requires validation by default today. The TL;DR for this code is:

Add the following to your Nerves project's target.exs or config.exs:

config :nerves_runtime, startup_guard_enabled: true

Add then add the following to your project's rel/vm.args.eex:

## Require an initialization handshake within 10 minutes
-env HEART_INIT_TIMEOUT 600

[Copilot]

Pull Request Overview

This PR introduces a minimal firmware validator called StartupGuard that monitors system startup and automatically validates firmware for simple use cases. The validator waits for all OTP applications to start, then validates unvalidated firmware, with a 15-minute timeout and heart callback protection against hangs.

• Adds Nerves.Runtime.StartupGuard module with heart integration for automatic firmware validation
• Enhances firmware validation status checking with new firmware_validation_status/0 function
• Improves KV module error handling to prevent crashes when the GenServer isn't running

Reviewed Changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
lib/nerves_runtime/startup_guard.ex	New module implementing firmware validation with heart callbacks and application startup monitoring
lib/nerves_runtime/application.ex	Conditionally starts StartupGuard based on configuration
lib/nerves_runtime.ex	Adds firmware_validation_status/0 function and get_expected_started_apps/0 helper
lib/nerves_runtime/kv.ex	Adds safe_call wrapper to handle GenServer crashes gracefully
test/nerves_runtime/startup_guard_test.exs	Comprehensive test suite for StartupGuard functionality
test/nerves_runtime_test.exs	Updates tests to use new firmware_validation_status/0 function
test/test_helper.exs	Adds Mimic setup for mocking Erlang modules
mix.exs	Adds Mimic dependency and custom test runner for Erlang module mocking
README.md	Updates documentation to reflect new StartupGuard functionality and configuration

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated 6 comments.

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated 5 comments.

---

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[Copilot]

The Logger.configure(level: :error) line was removed but not replaced. This may cause tests to output more verbose logs than intended. Consider adding it back or replacing it with an appropriate log level configuration for tests.

Suggested change

	ExUnit.start()
	ExUnit.start()
	Logger.configure(level: :error)

[Copilot]

The run_test/1 function doesn't return the exit status from System.cmd/3. When mix test is run with ERL_FLAGS not set, the function will return the result of System.cmd/3 (a tuple {output, exit_code}), but Mix tasks expect specific return values. Consider handling the exit code appropriately, such as calling System.halt(exit_code) if the command fails.

Suggested change

	System.cmd("mix", ["test" | options],
	into: IO.stream(:stdio, :line),
	env: [{"ERL_FLAGS", "-nostick"}]
	)
	{_output, exit_code} =
	System.cmd("mix", ["test" | options],
	into: IO.stream(:stdio, :line),
	env: [{"ERL_FLAGS", "-nostick"}]
	)
	System.halt(exit_code)