Add a root shell on UART console

Until I implement some kind of self-provisioning process for these
machines (supposing I ever do), I need a way to log in and
configure/troubleshoot.  I don't think there's any particular security
concern by having an auto-logged-in root shell on the UART console, as
accessing it needs physical access to the machine.

airplaypi_defconfig.in

@@ -6,6 +6,7 @@ BR2_GLOBAL_PATCH_DIR="board/raspberrypi/patches"
 BR2_DOWNLOAD_FORCE_CHECK_HASHES=y
 BR2_TARGET_GENERIC_HOSTNAME="airplaypi"
 BR2_INIT_SYSTEMD=y
+BR2_ROOTFS_OVERLAY="$(WORKSPACE)/rootfs-overlay"
 BR2_ROOTFS_POST_IMAGE_SCRIPT="$(BR2_EXTERNAL_AIMEEOS_PATH)/board/raspberrypi3/post-image.sh"
 BR2_LINUX_KERNEL=y
 BR2_LINUX_KERNEL_CUSTOM_TARBALL=y

rootfs-overlay/usr/lib/systemd/system-preset/50-airplaypi.preset

@@ -0,0 +1 @@
+enable root-shell@.service ttyAMA0

rootfs-overlay/usr/lib/systemd/system/root-shell@.service

@@ -0,0 +1,26 @@
+[Unit]
+Description=root shell on %I
+After=sshd.service
+Conflicts=shutdown.target
+Conflicts=getty@%i.service serial-getty@%i.service
+
+[Service]
+Type=idle
+Environment=TERM=linux
+ExecStart=/bin/sh
+Restart=always
+RestartSec=0
+StandardInput=tty
+TTYPath=/dev/%I
+TTYReset=yes
+TTYVHangup=yes
+KillMode=process
+IgnoreSIGPIPE=no
+KillSignal=SIGHUP
+
+# Unset locale for the console getty since the console has problems
+# displaying some internationalized messages.
+UnsetEnvironment=LANG LANGUAGE LC_CTYPE LC_NUMERIC LC_TIME LC_COLLATE LC_MONETARY LC_MESSAGES LC_PAPER LC_NAME LC_ADDRESS LC_TELEPHONE LC_MEASUREMENT LC_IDENTIFICATION
+
+[Install]
+WantedBy=multi-user.target