sstate/sstatesig: Add populate_lic to list of arch invariant sstate tasks

sstate/sstatesig: Add populate_lic to list of arch invariant sstate tasks

Like fetch, unpack and patch, populate_lic doesn't vary between different
archs so we should mark it as such. This means better sstate cache reuse
with fewer duplicate files as well as less confusing sstate debugging.

sstatesig also needs to account for the fact BPN is used for sstate files
in these cases.

(From OE-Core rev: 3d59d0bed756f64d0092caa3892239c779c4a341)

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