bitbake.conf: Add BB_CURRENTTASK to BB_HASHEXCLUDE

Tasks shouldn't vary dependning on the value of BB_CURRENTTASK. They
happen not to due to when bitbake sets this but to fix other issues,
bitbake needs to set it earlier. Therefore exclude from hashes
globally.

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