Add support for Raspberry Pi 3

[trickeydan]

Packages on ALARM tarball

``` acl 2.3.1-1 archlinuxarm-keyring 20140119-1 argon2 20190702-3 attr 2.5.1-1 audit 3.0.1-1 base 2-2 bash 5.1.004-1 bzip2 1.0.8-4 ca-certificates 20181109-4 ca-certificates-mozilla 3.64-1 ca-certificates-utils 20181109-4 coreutils 8.32-1 crda 4.14-4 cryptsetup 2.3.5-4 curl 7.76.1-1 dbus 1.12.20-1 device-mapper 2.03.11-5 dhcpcd 9.4.0-1 dialog 1:1.3_20210324-1 diffutils 3.7-3 dnssec-anchors 20190629-3 e2fsprogs 1.46.2-1 expat 2.3.0-1 file 5.40-2 filesystem 2021.01.19-1 findutils 4.8.0-1 firmware-raspberrypi 20210224-1 gawk 5.1.0-1 gcc-libs 10.2.0-1 gettext 0.21-1 glib2 2.68.1-1 glibc 2.32-2 gmp 6.2.1-1 gnupg 2.2.27-1 gnutls 3.7.1-1 gpgme 1.15.1-1 grep 3.6-1 gzip 1.10-3 haveged 1.9.14-1 hwids 20201207-1 iana-etc 20210202-1 icu 69.1-1 iproute2 5.12.0-1 iptables 1:1.8.7-1 iputils 20210202-1 iw 5.9-1 json-c 0.15-1 kbd 2.4.0-2 keyutils 1.6.3-1 kmod 28-1 krb5 1.19.1-1 ldns 1.7.1-2 less 1:581.2-1 libarchive 3.5.1-1 libassuan 2.5.5-1 libcap 2.49-1 libcap-ng 0.8.2-1 libcroco 0.6.13-2 libedit 20210419_3.1-1 libelf 0.183-3 libffi 3.3-4 libgcrypt 1.9.3-1 libgpg-error 1.42-1 libidn2 2.3.0-1 libksba 1.4.0-2 libldap 2.4.58-2 libmnl 1.0.4-3 libnetfilter_conntrack 1.0.8-1 libnfnetlink 1.0.1-4 libnftnl 1.1.9-1 libnghttp2 1.43.0-1 libnl 3.5.0-2 libp11-kit 0.23.22-1 libpcap 1.10.0-1 libpsl 0.21.1-1 libsasl 2.1.27-3 libseccomp 2.5.1-2 libsecret 0.20.4-1 libssh2 1.9.0-3 libtasn1 4.16.0-1 libtirpc 1.3.1-1 libunistring 0.9.10-3 libxcrypt 4.4.20-1 libxml2 2.9.10-9 licenses 20200427-1 linux-api-headers 5.10.13-1 linux-firmware 20210426.fa0efef-1 linux-raspberrypi 5.10.33-2 lz4 1:1.9.3-1 mkinitcpio 30-1 mkinitcpio-busybox 1.32.1-3 mpfr 4.1.0-2 nano 5.7-1 ncurses 6.2-2 net-tools 2.10-1 netctl 1.24-1 nettle 3.7.2-1 npth 1.6-3 openresolv 3.12.0-1 openssh 8.6p1-1 openssl 1.1.1.k-1 p11-kit 0.23.22-1 pacman 5.2.2-3 pacman-mirrorlist 20210307-1 pam 1.5.1-1 pambase 20200721.1-2 pciutils 3.7.0-1 pcre 8.44-1 pcre2 10.36-1 pinentry 1.1.1-1 popt 1.18-1 procps-ng 3.3.17-1 psmisc 23.4-1 raspberrypi-bootloader 20210430-1 raspberrypi-bootloader-x 20210430-1 raspberrypi-firmware 20210430-1 readline 8.1.0-2 sed 4.8-1 shadow 4.8.1-4 sqlite 3.35.5-1 systemd 248-5 systemd-libs 248-5 systemd-sysvcompat 248-5 tar 1.34-1 tzdata 2021a-1 util-linux 2.36.2-1 util-linux-libs 2.36.2-1 vi 1:070224-4 which 2.21-5 wireless-regdb 2020.11.20-1 wireless_tools 30.pre9-3 wpa_supplicant 2:2.9-8 xz 5.2.5-1 zlib 1:1.2.11-4 zstd 1.4.9-1 ```

[trickeydan]

RPi packages seem to be:

firmware-raspberrypi 20210224-1
linux-raspberrypi 5.10.33-2
raspberrypi-bootloader 20210430-1
raspberrypi-bootloader-x 20210430-1
raspberrypi-firmware 20210430-1

[trickeydan]

Completed in #14