sstate: Ensure sstate searches update file mtime

sstate: Ensure sstate searches update file mtime

Commands like "bitbake XXX -S printdiff" search for sstate files but don't download
them. This means that local files aren't touched as the download code would do, meaning
the sstate cleanup scripts can delete them. This can then lead to obtuse build failures.

Have the search code touch local files in the same way as the main code paths would to
avoid these files disappearing.

Move the function to a common touch() function in lib/oe instead of duplicating code.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>