scripts/bitbake-prserv-tool: Fix to work with memres bitbake

scripts/bitbake-prserv-tool: Fix to work with memres bitbake

Deleting data files whilst bitbake is running isn't a great idea.

Make sure memory resident bitbake is stopped if present and make
the file deletions a little more specific and safer too while we're
here.

Note: The cache directory layout is different on older releases (some
are split level) so this won't be a direct backport to some.

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