kernel: stop using -exec rm for deleting files

kernel: stop using -exec rm for deleting files

Removing files from the source tree via find, exec and rm is not the
most efficient operation, due to (among other things) the many forked
processes.

If we use -delete, it saves a significant amount of time. But -delete
does not work with -prune (since it forces -depth). To maintain the
lib, tools and scripts source files, we can hide them temporarily,
skip their hidden directories and then finally restore them.

Time for install before this change:

 real    2m48.563s
 user    0m35.220s
 sys     0m33.036s

Time for install after this change:

 real    1m21.301s
 user    0m33.160s
 sys     0m28.388s

We could further speed this up by using inline perl to delete the files,
but that complexity is avoided for now.

(From OE-Core rev: 01932d6bbc71e86fd903097b5339e91f76846388)

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>