Add instructions for compiling devkit kernel
From Matrix:
mdz0: I just compiled a kernel package using https://source.puri.sm/Librem5/linux-emcraft by adjusting the instructions here https://source.puri.sm/Librem5/image-builder/blob/master/build/kernel.txt
08:49
sudo apt-get install gcc-aarch64-linux-gnu build-essential crossbuild-essential-arm64
cp arch/arm64/configs/librem5-evk_defconfig .config
make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- menuconfig
make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- deb-pkg