add RTC

[gkuznik]

No description provided.

[Copilot]

Pull request overview

This PR adds RTC support to the kernel and STM32L4 machine layer so the kernel can read/write wall-clock time and persist a small amount of RTC-backed state across resets.

Changes:

• Add RTC initialization, raw RTC accessors, and backup-register access in the STM32L4 C interface.
• Expose RTC APIs through the machine abstraction and implement them for native and stub Cortex-M backends.
• Add kernel-side time helpers plus a new time thread that is started during kernel initialization.

Reviewed changes

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

Show a summary per file

File	Description
src/time.rs	Adds kernel RTC helpers, Unix/RTC conversion logic, and the new time thread.
src/lib.rs	Starts the new time thread during kernel initialization.
machine/cortex-m/st/stm32l4/interface/export.h	Exposes new RTC and backup-register symbols to Rust bindings.
machine/cortex-m/st/stm32l4/interface/clock.c	Initializes RTC hardware, selects RTC clock source, and implements RTC read/write helpers.
machine/cortex-m/src/stub.rs	Adds stub RTC methods for the test/non-hardware backend.
machine/cortex-m/src/native.rs	Wires the new RTC bindings into the native Cortex-M machine implementation.
machine/api/src/lib.rs	Extends the machine trait with RTC and backup-register operations.

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[thomasw04]

If I understand correctly, you assume that every HAL exposes an rtc_raw function that returns a BCD-encoded time. I would not make this the public API. I think it makes more sense to expose POSIX time as universal format and make the conversion HAL/driver defined.