We need to copy this set, not modify the original else all kinds
of weird and bad things break, mostly from circular references.
We'll not go into how much sleep I lost tracking down the fallout
from this.
(Bitbake rev: 49927546d2b306830c98f6f9da4a6ad828f6a3a6)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>