Build AlpineLinux 3.7 aarch64 for the RaspberryPi 3

BuildMe AlpineLinux chroot env aarch64

# The following steps will show you how to deploy a AlpineLinux 3.7 aarch64 for the RaspberryPi 3 from a chrooted environment.
#
# This image was built using a Gentoo aarch64 HOST. 
# 
# Steps for building the kernel are not provided in this guide. 
#
# However I have used this wiki page as guidance to build mine https://wiki.gentoo.org/wiki/Raspberry_Pi_3_64_bit_Install .
#
# I am sure you will find lots of weirdness here along with many things that can be improved. 
#
# If so and care to help, feel free to email your feedbacks to pietro.sammarco@googlemail.com  .

# mkdir alpine 
# cd alpine

- As we speak latest version is apk-tools-static-2.9.1-r0. If you get 404 you will need to change it with actual version.

# wget http://uk.alpinelinux.org/alpine/latest-stable/main/aarch64/apk-tools-static-2.9.1-r0.apk
# tar -xzf apk-tools-static-*.apk

- Pulling the "stage3" binaries;

# ./sbin/apk.static -X http://uk.alpinelinux.org/alpine/latest-stable/main --arch aarch64 -U --allow-untrusted --root . --initdb add alpine-base

- Mounting the chrooted environment;

# mount -t proc none proc
# mount -o bind /sys sys
# mount -o bind /dev dev
# cp /etc/resolv.conf etc/

# chroot . /bin/sh -l
# source /etc/profile
# export PS1="(alpine) ${PS1}"

- Adding the APK repo;

# echo "http://uk.alpinelinux.org/alpine/latest-stable/main" > /etc/apk/repositories

- Installing few essential packages;

# apk update && apk upgrade
# apk add chrony
# apk add openssh openssh-server
# apk add wpa_supplicant
# apk add sudo

- Enabling a bunch of rc services;
# rc-update add devfs sysinit
# rc-update add dmesg sysinit
# rc-update add mdev sysinit
# rc-update add modules boot
# rc-update add sysctl boot
# rc-update add hostname boot
# rc-update add bootmisc boot
# rc-update add syslog boot
# rc-update add mount-ro shutdown
# rc-update add killprocs shutdown
# rc-update add savecache shutdown
# rc-update add udhcpd boot
# rc-update add swclock boot
# rc-update add sshd default
# rc-update add wpa_supplicant boot
# rc-update add local default
# rc-update add ntpd default

- Your /etc/fstab should look similar to this (this is a simple rootfs plus /boot layout);

/dev/mmcblk0p1          /boot           vfat            noauto,noatime  1 2
/dev/mmcblk0p2          /               ext4            noatime         0 1

- This create a 50mb compressed zram swap disk. The script goes into /etc/local.d/zram.start;

#
#!/bin/sh

modprobe zram

SIZE=50
echo $((50*1024*1024)) > /sys/block/zram0/disksize

mkswap /dev/zram0

swapon /dev/zram0 -p 20
#

# chmod +x /etc/local.d/zram.start

- This will unmount the zram swap disk prior to shutdown. Script goes into /etc/local.d/zram.stop;

#
#!/bin/sh

swapoff /dev/zram0

echo 1 > /sys/block/zram0/reset

modprobe -r zram
#

# chmod +x /etc/local.d/zram.stop

- Add the following to /etc/network/interfaces;

auto eth0
iface eth0 inet dhcp
 
auto wlan0
iface wlan0 inet dhcp 

- Specifying a server for ntpd to work; 

# echo 'NTPD_OPTS="-4 -N -p pool.ntp.org"' > /etc/conf.d/ntpd

- Create a new user;

# adduser alpine -s /bin/sh -h /home/alpine
 
- In order for the Wireless NIC to work you will need to install the linux-firmware package which contains the Broadcom drives;

# apk update && apk add linux-firmware
- Since /lib/firmware/brcm is only really needed, if you want to save space you could backup the brcm folder, remove the linux-firmware package and place back the brcm folder to /lib/firmware/ .

- Load the Broadcom modules on boot
# echo -e 'b43 \nbrcmfmac' >> /etc/modules

- Add the following entries to /boot/cmdline.txt. Make sure root= matches the / partition;
root=/dev/mmcblk0p2 rootfstype=ext4 fsck.repair=yes rootwait

- If you made it this far you should now have a minimal Alpinelinux 3.7 aarch64 rootfs. Though you might also want a kernel to actually boot the system up...  
- Here is the /boot folder https://goo.gl/hKECev which contains kernel 4.10.17-v8+, blobs for the Wireless NIC the bootcode as well as the usual overlays for the RPI3.

- Basically just extract it directly to the /boot partition of the sdcard;

# tar xvjf rpi3-alpine-boot.tar.bz2 -C /path/of/whatever/boot

- Installation guide needs to be revised, though aside from some nonesense it should work just fine https://pastebin.com/raw/RfNQ5tXv