[BUG] cmake-based build should not modify CMAKE_CURRENT_SOURCE_DIR

[yamt]

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

• I have verified before submitting the report.

[xiaoxiang781216]

@xuxin930 please look at this problem.

[xuxin930]

Yes @yamt very nice advice.

most of the modifications to the source tree now is
fetching data from external repositories.

considering the possibility of reusing these external repos.
Is it acceptable to keep them?

[yamt]

a problem is that sometimes it prevents users from using multiple build dirs with a single source tree.
eg. when fetched data is configuration-dependent. (different versions, different local patches, etc)

IMO, it's simpler to put downloaded files under the build dir.

if we want to to have a shared cache of downloaded files, maybe it's cleaner to use a dedicated dir, instead of the source dir.