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.

(From meta-openpower rev: 4b5603b4426442973f29392ee4d0955f48680fac)

Change-Id: I1d187662576dea93550d21cf7031fbc663610fe7
Signed-off-by: Alexander Filippov <a.filippov@yadro.com>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/meta-openpower/recipes-phosphor/network/first-boot-set-mac/first-boot-set-mac@.service b/meta-openpower/recipes-phosphor/network/first-boot-set-mac/first-boot-set-mac@.service
index 8986efd..3ff5fc1 100644
--- a/meta-openpower/recipes-phosphor/network/first-boot-set-mac/first-boot-set-mac@.service
+++ b/meta-openpower/recipes-phosphor/network/first-boot-set-mac/first-boot-set-mac@.service
@@ -1,5 +1,6 @@
 [Unit]
 Description=Init BMC MAC address for NIC %I
+ConditionPathExists=!/var/lib/first-boot-set-mac/%i
 Wants=mapper-wait@-xyz-openbmc_project-inventory.service
 After=mapper-wait@-xyz-openbmc_project-inventory.service
 Wants=mapper-wait@-xyz-openbmc_project-network-%i.service