package/petitboot: properly shut down before kexec

It's good practice to unmount filesystems and gracefully terminate
running services before running "kexec -e". So when a boot option has
been chosen from the petitboot menu, poke init to shut the system down
and kexec the new kernel.

One benefit to us in particular is that when pb-console is killed, it
notifies the user that we're booting:

  trap 'reset; echo "SIGTERM received, booting..."; sleep 2' SIGTERM

This terminal reset is also useful, exiting the ncurses visual mode so
subsequent boot output is raw rather than being confined to the window
set up for the petitboot menu.

Currently we assume busybox init (using the bb-kexec-reboot script
included with petitboot), but do not add an accompanying entry
in the inittab to run kexec, so things aren't working.

Add a new script kexec-restart that does the right thing for all init
systems that Buildroot supports (busybox, sysv, openrc, systemd). OpenRC
and systemd have an upstream way to do a kexec-shutdown. For busybox and
sysv, we add a line to inittab that does kexec -f -e (after the shutdown
scripts have run). Finally, for other cases (where there is no
recognized init system) directly use kexec -f -e.

Signed-off-by: Reza Arbab <arbab@linux.ibm.com>
[Arnout:
 - install kexec-restart in /usr/libexec/petitboot, where it was before;
 - determine init system statically based on config instead of dynamically.
]
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>

package/petitboot/kexec-restart.in

@@ -0,0 +1,8 @@
+#!/bin/sh
+
+@KEXEC_COMMAND@
+
+while :
+do
+	sleep 1
+done

package/petitboot/petitboot.mk

@@ -30,7 +30,7 @@ PETITBOOT_CONF_OPTS = \
 	--without-twin-x11 \
 	$(if $(BR2_PACKAGE_BUSYBOX),--enable-busybox,--disable-busybox) \
 	HOST_PROG_KEXEC=/usr/sbin/kexec \
-	HOST_PROG_SHUTDOWN=/usr/libexec/petitboot/bb-kexec-reboot
+	HOST_PROG_SHUTDOWN=/usr/libexec/petitboot/kexec-restart
 
 # HPA and Busybox tftp are supported. HPA tftp is part of Buildroot's tftpd
 # package.
@@ -55,11 +55,37 @@ else
 PETITBOOT_CONF_OPTS += --without-fdt
 endif
 
+ifeq ($(BR2_INIT_BUSYBOX),y)
+# inittab "restart" runlevel entry runs kexec
+PETITBOOT_KEXEC_COMMAND = /bin/kill -QUIT 1
+define PETITBOOT_BUSYBOX_INITTAB
+	grep -q kexec $(TARGET_DIR)/etc/inittab || \
+		printf "\nnull::restart:/usr/sbin/kexec -f -e\n" >> $(TARGET_DIR)/etc/inittab
+endef
+PETITBOOT_TARGET_FINALIZE_HOOKS += PETITBOOT_BUSYBOX_INITTAB
+else ifeq ($(BR2_INIT_SYSV),y)
+# inittab runlevel 6 entry runs kexec
+PETITBOOT_KEXEC_COMMAND = /sbin/shutdown -r now
+define PETITBOOT_SYSV_INITTAB
+	grep -q kexec $(TARGET_DIR)/etc/inittab || \
+		$(SED) 's~^reb0:.*~reb0:6:wait:/usr/sbin/kexec -f -e~' $(TARGET_DIR)/etc/inittab
+endef
+PETITBOOT_TARGET_FINALIZE_HOOKS += PETITBOOT_SYSV_INITTAB
+else ifeq ($(BR2_INIT_OPENRC),y)
+PETITBOOT_KEXEC_COMMAND = /sbin/openrc-shutdown --kexec now
+else ifeq ($(BR2_INIT_SYSTEMD),y)
+PETITBOOT_KEXEC_COMMAND = /usr/bin/systemctl kexec
+else # BR2_INIT_NONE
+PETITBOOT_KEXEC_COMMAND = /usr/sbin/kexec -f -e
+endif
+
 PETITBOOT_GETTY_PORT = $(patsubst %,'%',$(call qstrip,$(BR2_PACKAGE_PETITBOOT_GETTY_PORT)))
 
 define PETITBOOT_POST_INSTALL
-	$(INSTALL) -D -m 0755 $(@D)/utils/bb-kexec-reboot \
-		$(TARGET_DIR)/usr/libexec/petitboot/bb-kexec-reboot
+	$(INSTALL) -D -m 0755 $(PETITBOOT_PKGDIR)/kexec-restart.in \
+		$(TARGET_DIR)/usr/libexec/petitboot/kexec-restart
+	$(SED) 's~@KEXEC_COMMAND@~$(PETITBOOT_KEXEC_COMMAND)~' \
+		$(TARGET_DIR)/usr/libexec/petitboot/kexec-restart
 	$(INSTALL) -D -m 0755 $(@D)/utils/hooks/01-create-default-dtb \
 		$(TARGET_DIR)/etc/petitboot/boot.d/01-create-default-dtb
 	$(INSTALL) -D -m 0755 $(@D)/utils/hooks/90-sort-dtb \
@@ -77,6 +103,7 @@ define PETITBOOT_POST_INSTALL
 	mkdir -p $(TARGET_DIR)/usr/share/udhcpc/default.script.d/
 	ln -sf /usr/sbin/pb-udhcpc \
 		$(TARGET_DIR)/usr/share/udhcpc/default.script.d/
+
 endef
 
 PETITBOOT_POST_INSTALL_TARGET_HOOKS += PETITBOOT_POST_INSTALL