[BUG] cmake-based build should not modify CMAKE_CURRENT_SOURCE_DIR

### Description / Steps to reproduce the issue

it seems that cmake-based build sometimes modifies files under CMAKE_CURRENT_SOURCE_DIR.
eg. it expands littlefs source tree to fs/littlefs/littlefs.
it should be done under CMAKE_BINARY_DIR instead.
we should treat CMAKE_CURRENT_SOURCE_DIR read-only.

### On which OS does this issue occur?

[OS: Mac]

### What is the version of your OS?

macOS 14.7

### NuttX Version

master

### Issue Architecture

[Arch: all]

### Issue Area

[Area: Build System]

### Verification

- [X] I have verified before submitting the report.