Clean up tools build

This MR aims to reduce the amount of custom and repeated logic in the files under debian/rules.d.

1. Remove the clean targets, which have been obsolete since the linux-tools source package was merged into linux.
2. Introduce and use the srcdir variable for upstream source directory. We need this value in many places and are currently writing it in a much longer way.
3. Split library and tools builds in debian/rules.d/tools/thermal into subdirectories.
4. Introduce and use variables for invoking upstream makefiles. These replace a lot of similar (but not identical) variable definitions that are currently used for invoking upstream non-Kbuild makefiles.
5. Build user-space tools with Kbuild where possible. This does the same thing for most of the tools that can be built with Kbuild and removes duplicate rules from our makefiles.
6. [x86] d/rules.d/tools/power/x86: Use upstream makefiles to build programs. This uses (3) to eliminate the need for duplicate definitions in our makefiles.