Refactor
Use RPI_MODEL and SET_ARCH to define all board relevant settings, crosscompilers, architectures etc...
- SET_ARCH=32 is default and used for all boards in 32bit mode
- SET_ARCH=64 and RPI_MODEL=3 or 3P activate 64bit mode!
- no need to set CROSS_COMPILE, KERNEL_IMAGE, RELEASE_ARCH, KERNEL_DEFCONFIG, KERNEL_ARCH, QEMU_BINARY
- build kernel is now a default. i kicked out old kernel v3 and jessie support! they're too old. set buster as default
- autodetect if program is running on 3142 => assume it is apt-cacher-ng and set APT_PROXY
- next pull includes sakaki kernel for RPI3 and RPi3B+ => see
https://github.com/sakaki-/bcmrpi3-kernel-bis