Updating the value of OVERRIDES whenever it changes turns out to be
extremely expensve/pointless. Instead, clear its value and re-establish
the value when we're going to use it.
This gives significant speed back.
(Bitbake rev: 41cf8d0c92d2d8a33fdad0921e424a0024914be1)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>