The value of PARALLEL_MAKE shouldn't affect sstate checksums. In general it
doesn't as the value is injected as a task override and the sstate checksum
doesn't track these. That isn't the case for linux-yocto in particular so
adding it the list of variables to ignore is useful.
(From OE-Core rev: a0fbbd7ba979b8aaee701e0997115f89b361b920)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>