tooling: split Kbuild-related rules from the main Makefile

tooling: split Kbuild-related rules from the main Makefile

Split all parts of the main Makefile, that are only related to the
kernel build-system, into their own file (Kbuild). When invokes, this
file will be search first for additional rules by the kernel
build-system.

The remaining rules in our own root-Makefile will not be seen by the
kernel build-system anymore and thus are easier to extend.

Also, change some variable-names to fit those suggested by the
kernel-documentation (Documentation/kbuild/modules.txt).

Signed-off-by: Benjamin Block <bebl@mageta.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>