base/bitbake.conf: Introduce UNPACKDIR

base/bitbake.conf: Introduce UNPACKDIR

Having the unpack directory hardcoded to WORKDIR makes it really hard to
make any changes to the unpack process to try and allow for cleanup for example.

As a first step toward unraveling the intertwined location usages, add a
variable, UNPACKDIR which is where the fetcher is asked to unpack fetched
sources. It defaults to the existing value of WORKDIR at this point.

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