hw kernel trace

[Martin Stein]

Although this feature is not meant to be pushed onto genodelabs/master,
i've observed that it's often useful for me to have some tools in
base-hw that allow for typical tracing anytime the kernel is called.
Thus i started a branch
https://github.com/m-stein/genode/tree/hw_kernel_trace that provides
such tools in form of simple commits, wich one can cherry-pick to its
current issue branch just in case. I'll try to keep this branch
up-to-date with genodelabs/master manually.