Add Systemd init system support

Fixes #77

Makefile

@@ -2,7 +2,7 @@ PREFIX ?= /
 
 CLOUDS = $(sort $(notdir $(patsubst %/,%,$(wildcard lib/tiny-cloud/cloud/*/))))
 
-SUBPACKAGES = core openrc $(CLOUDS)
+SUBPACKAGES = core init $(CLOUDS)
 
 .PHONY: check install $(SUBPACKAGES)
 
@@ -22,9 +22,13 @@ core:
 	install -Dm755 -t "$(PREFIX)"/usr/sbin \
 		sbin/tiny-cloud
 
-openrc:
-	install -Dm755 -t "$(PREFIX)"/etc/init.d \
-		dist/openrc/*
+init:
+	[ $(command -v rc-status) ] && \
+		install -Dm755 -t "$(PREFIX)"/etc/init.d \
+			dist/openrc/*
+	[ $(command -v systemctl) ] && \
+		install -Dm755 -t "$(PREFIX)"/etc/systemd/system \
+			dist/systemd/*
 
 $(CLOUDS):
 	install -Dm644 -t "$(PREFIX)"/usr/lib/tiny-cloud/cloud/$@ \

dist/systemd/tiny-cloud-boot.service

@@ -0,0 +1,10 @@
+[Unit]
+Description=Tiny Cloud Bootstrap - boot phase
+
+[Service]
+Type=oneshot
+ExecStart=tiny-cloud boot
+Restart=no
+
+[Install]
+WantedBy=multi-user.target

dist/systemd/tiny-cloud-early.service

@@ -0,0 +1,11 @@
+[Unit]
+Description=Tiny Cloud Bootstrap - early phase
+After=network.target
+
+[Service]
+Type=oneshot
+ExecStart=tiny-cloud early
+Restart=no
+
+[Install]
+WantedBy=multi-user.target

dist/systemd/tiny-cloud-final.service

@@ -0,0 +1,11 @@
+[Unit]
+Description=Tiny Cloud Bootstrap - final phase
+After=tiny-cloud-boot.service tiny-cloud-main.service tiny-cloud-early.service
+
+[Service]
+Type=oneshot
+ExecStart=tiny-cloud final
+Restart=no
+
+[Install]
+WantedBy=multi-user.target

dist/systemd/tiny-cloud-main.service

@@ -0,0 +1,12 @@
+[Unit]
+Description=Tiny Cloud Bootstrap - main phase
+After=network.target sshd.service
+
+[Service]
+Type=oneshot
+ExecStart=tiny-cloud main
+Restart=no
+
+[Install]
+WantedBy=multi-user.target
+