fs: add merge partitions support

[gneworld]

Summary

Usage example for merging the factory partition and reserve partition into the merge partition:
register_merge_blockdriver("/dev/merge", "/dev/factory", "/dev/reserve", NULL)

Impact

FS

Testing

test on bes platform

[nuttxpr]

[Experimental Bot, please feedback here]

No, this PR does not fully meet the NuttX requirements as described. While it provides a usage example and mentions testing, it lacks crucial information.

Here's a breakdown of missing information:

• Summary: Needs to explain why this change is necessary. Is it a new feature? Fixing a bug? Improving performance? What problem does it solve? It also lacks any mention of related issues.
• Impact: While it mentions "FS" (presumably filesystem), it needs to be far more specific. It needs to answer all the yes/no questions about user impact, build impact, hardware impact, documentation needs, security implications, and compatibility. Simply saying "FS" isn't helpful.
• Testing: "test on bes platform" is insufficient. It needs to specify the host OS, CPU, compiler used for building, and the specific target architecture, board, and configuration. Most importantly, it lacks the required "before" and "after" testing logs. Without these logs, there's no evidence provided that the change actually works or improves anything.

In short, the PR needs significantly more detail to be considered complete.

[lupyuen]

Sorry I have to cancel the CI Check, there's a problem with CI Test:

• drivers/pipes: return after short write if buffer is full #14667 (comment)