first-boot-set-*: Use ConditionPathExists

Replaces the calls of `systemctl disable` with `ConditionPathExists='
in the systemd units files.

It will allow us to distinguish an admin created state and a runtime
created state.

Change-Id: Icde57c942166c40274eddc738ed1ed704eea80a4
Signed-off-by: Alexander Filippov <a.filippov@yadro.com>
diff --git a/recipes-phosphor/network/first-boot-set-mac/first-boot-set-mac.sh b/recipes-phosphor/network/first-boot-set-mac/first-boot-set-mac.sh
index da1b305..63a4ad7 100755
--- a/recipes-phosphor/network/first-boot-set-mac/first-boot-set-mac.sh
+++ b/recipes-phosphor/network/first-boot-set-mac/first-boot-set-mac.sh
@@ -79,4 +79,6 @@
 
 sync_mac $1
 
-systemctl --no-reload disable first-boot-set-mac@${1}.service
+# Prevent start at next boot time
+mkdir -p "/var/lib/first-boot-set-mac"
+touch "/var/lib/first-boot-set-mac/${1}"