staging: Improve fixup processing code

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.

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