utils: Add option to run root shell on console

Until I develop a captive portal-based mechanism for initially
configuring the WiFi on Raspberry Pi machines, the easiest way to do
initial provisioning is using the CLI via the serial console.  Since the
root account has no password, and I don't want to have to specify one in
downstream projects' configurations, I've added a configuration option
to enable automatically launching a root shell connected to the serial
console instead of a login prompt.  The security risks here are pretty
minimal, because someone would need phyiscal access to the device in
order to use this shell, in which case they could use any number of
other methods to get control of the system.

configs/aimeeos_qemu_aarch64_defconfig

@@ -11,4 +11,4 @@ BR2_LINUX_KERNEL_NEEDS_HOST_OPENSSL=y
 # BR2_PACKAGE_BUSYBOX is not set
 # BR2_TARGET_ROOTFS_TAR is not set
 AIMEEOS=y
-AIMEEOS_DEFAULT_ROOTFLAGS="systemd.mask=serial-getty@ttyAMA0 systemd.debug_shell systemd.default_debug_tty=ttyAMA0"
+BR2_PACKAGE_AIMEE_OS_ROOT_SHELL=y

package/aimee-os-utils/Config.in

@@ -8,3 +8,25 @@ config BR2_PACKAGE_AIMEE_OS_UTILS
     select BR2_PACKAGE_UTIL_LINUX_SWITCH_ROOT
     select BR2_PACKAGE_WGET
     select BR2_TARGET_GRUB2_INSTALL_TOOLS
+
+if BR2_PACKAGE_AIMEE_OS_UTILS
+
+config AIMEE_OS_ROOT_SHELL
+    bool "Spawn a root shell on the serial console by default"
+    help
+      With this option enabled, the system will boot up normally, with
+      a root shell connected to the default serial console.  This is
+      useful for initial provisioning and troubleshooting.
+
+if AIMEE_OS_ROOT_SHELL
+
+config AIMEE_OS_ROOT_SHELL_CONSOLE
+    string "Console name"
+    default ttyAMA0
+    help
+      Name of the console device where the root shell will be spawned.
+      Do not include the /dev prefix
+
+endif
+
+endif

package/aimee-os-utils/aimee-os-utils.mk

@@ -28,4 +28,20 @@ define AIMEE_OS_UTILS_INSTALL_INIT_SYSTEMD
 		$(TARGET_DIR)/usr/lib/systemd/network/90-default.network
 endef
 
+define AIMEE_OS_UTILS_ROOT_SHELL_INSTALL
+
+$(INSTALL) -D -m u=rw,go=r \
+	$(AIMEE_OS_UTILS_PKGDIR)/root-shell@.service \
+	$(TARGET_DIR)/usr/lib/systemd/system/root-shell@.service
+$(INSTALL) -d -m u=rwx,go=rx \
+	$(TARGET_DIR)/usr/lib/systemd/system-preset
+printf 'enable root-shell@.service %s\n' \
+	$(AIMEE_OS_ROOT_SHELL_CONSOLE) \
+	> $(TARGET_DIR)/usr/lib/systemd/system-preset/50-root-shell.preset
+endef
+
+ifeq ($(AIMEE_OS_ROOT_SHELL),y)
+AIMEE_OS_UTILS_INSTALL_INIT_SYSTEMD += $(AIMEE_OS_UTILS_ROOT_SHELL_INSTALL)
+endif
+
 $(eval $(generic-package))

package/aimee-os-utils/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