Makefile: initial version

From make help:

Build and optionally flash Pixel 6 images
The following make targets exist:
    help              this help
    clean             clean all generated files

    build             build all kernel artefacts
    images            generate all flashable images
    flash             flash and run images (potentially rebuilding them first)
    reflash           flash and run images (without rebuilding)
    boot              boot boot.img (potentially rebuilding images first)
    reboot            boot boot.img (without rebuilding or flashing)

    menuconfig        Linux menuconfig
    savedefconfig     Linux savedefconfig
    copydefconfig     savedefconfig and copy back into Linux source tree

    validate_dtbs     run various of the kernel's DTB validators

Note: For the boot or reboot targets to work, you should have run the
      flash target at least once first

There are also a few internal targets, but you should not need to
invoke them manually, hence they're not described here.

Note: While building, we use schedtool and ionice (if available) to
      increase interactive responsiveness of the system.

:-)