kconfig: expand the CLEANUP_INTERMEDIATE_FILES symbol description

Y-selecting CLEANUP_INTERMEDIATE_FILES negatively impacts tools/commands
that depend on the build artifacts being present. One such case is the
`west spdx` command used for Software Bill of Material generation.

If CLEANUP_INTERMEDIATE_FILES is y-selected, build files are removed from
the `cfgTarget.target.artifacts` list. This in turn causes the
`addBuildFile` function to nod add them to `pkg`, so `pkg.targetBuildFile`
is `None` in `scripts/west_commands/zspdx/walker.py`.

This commit expands the help message for the CLEANUP_INTERMEDIATE_FILES
symbol to inform the user of the possible negative impact of selecting this
symbol.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>