Since get_unihash uses taskhash as a key internally, changing it means
different bebahour when locked sigs are active verses not active. Under
corner cases this leads to a signature mismatch.
Avoid this by by adding a wrapper for the place its externally exposed
and then not changing the internals.
(From OE-Core rev: 23b7484d326cf7fac013384598d7745a042eaa75)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>