kernel: save $kerndir/tools and $kerndir/lib from pruning

The kernel source tree in the sysroot has all unecessary source
code removed. The existing use case is to support module building
out of the sysroot, but as more toolsa are moved into the kernel
tree itself there are new use cases for the kernel sysroot source.

To avoid putting dependencies on the kernel, and to be able to
individually build and package these tools out of the source tree,
we can save $kerndir/tools and $kernddir/lib from being removed.
This enables tools like perf to be built our of the kernel source
in the sysroot, without significantly increasing the amount of
source in the sysroot.

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>