rootfs: Add custom systemd presets

Instead of manually managing symlinks to enable units, we can define a
system preset that lists which units we want disabled.  Units that have
in `Install` section are automatically enabled.

rootfs/overlay/usr/lib/systemd/system-preset/70-jenkinsagent.preset

@@ -0,0 +1,10 @@
+enable htop.service
+
+disable cups-lpd.socket
+disable cups.service
+disable cups.socket
+disable cups.path
+
+disable systemd-networkd.service
+disable systemd-resolved.service
+disable systemd-timesyncd.service

rootfs/overlay/usr/lib/systemd/system/htop.service

@@ -17,3 +17,6 @@ ProtectKernelTunables=true
 ProtectKernelModules=true
 CapabilityBoundingSet=
 NoNewPrivileges=true
+
+[Install]
+WantedBy=multi-user.target

rootfs/overlay/usr/lib/systemd/system/multi-user.target.wants/htop.service

@@ -1 +0,0 @@
-../htop.service

rootfs/post-fakeroot.sh

@@ -1,9 +1,2 @@
 #!/bin/sh
 
-"${HOST_DIR}"/bin/systemctl --root="${TARGET_DIR}" disable \
-    cups-lpd.socket \
-    cups.service \
-    cups.socket \
-    systemd-networkd.service \
-    systemd-resolved.service \
-    --