bitbake.conf: Account for older versions of bitbake

bitbake.conf: Account for older versions of bitbake

Older versions of bitbake (prior to 1.42) don't expose the BB_UNIHASH
variable which is being used by sstate. For compatibility with these
older versions of bitbake, set BB_UNIHASH to BB_TASKHASH (which is the
value it should be for non-hash equivalent aware signature generators).
if bitbake hasn't already set it.

[YOCTO #13314]

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>