[OE-core] sysroots, wic-tools and rm_work

[Kristian Amlie]

It seems like wic doesn't work together with rm_work, because of its
dependency on wic-tools. The reason is that building wic-tools will
result in rm_work removing the very sysroot that wic needs.

It's easy to fix locally by adding

  RM_WORK_EXCLUDE = "wic-tools"

however, I think this deserves a more permanent fix, and I'm not sure
which approach is best. We could assign directly to that variable, but I
would assume that anyone that uses it will assign directly to it, and
breaking wic all over a again. Or perhaps we could add an exception
directly to rm_work.bbclass? What's best practice in a case like this?

-- 
Kristian