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-hostname/first-boot-set-hostname.service b/recipes-phosphor/network/first-boot-set-hostname/first-boot-set-hostname.service
index e58a046..22bb80c 100644
--- a/recipes-phosphor/network/first-boot-set-hostname/first-boot-set-hostname.service
+++ b/recipes-phosphor/network/first-boot-set-hostname/first-boot-set-hostname.service
@@ -1,5 +1,6 @@
 [Unit]
 Description=Init BMC Hostname
+ConditionPathExists=!/var/lib/first-boot-set-hostname
 Wants=op-vpd-parser.service
 After=op-vpd-parser.service