local.conf.sample: Weakly set BB_DISKMON_DIRS

For various reasons we need to be able to set and override this from
auto.conf on our test infrastructure. We have tried forcing the variable
but this then breaks other selftests. In the interests of not complicating
things further and needing to modify the tests across releases, weaken
the default assignment.

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