bitbake.conf: whitelist proxy variables in config hash

bitbake.conf: whitelist proxy variables in config hash

We don't need these variables incorporated in the config hash (i.e. we
don't need to reparse if they change).

This fixes an issue with the extensible SDK where the cache generated
at installation time not to be used afterwards due to the way we're
allowing through the proxy variables into the install environment (such
that they get set to "" if not set externally).

(From OE-Core rev: 3dc7c58b7b3224f3ed2d59d95313e70e4c57d7c4)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>