Makefile: add a target to (cross-)compile Linux

By moving away from a single config file, and instead maintain a separate list of CONFIG_* options, we can achieve two things:

1. make it easy to apply these options on different architectures
2. Avoid duplicating the instructions of how to compile the kernel between the README and config/linux_config_x86_64.sample.

With this step done, this commit also adds a new Makefile target that simplifies the compilation of Linux, and even cross-compilation!