staging: Improve fixup processing code
With the fixes to other parts of multilib, it was found that the fixup code's
assumptions about the recipe sysroot were incorrect. We need to use the value
calculated earlier in the function.
It turns out there is a rather neat way to do this which cleans up the code
as an added bonus.
(From OE-Core rev:
2c1978fe1a5b72167c49010fbdd39a9e2eefdef8)
(From OE-Core rev:
ef7e612e54d7629518a13ed3b1c2967bf70347a2)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
