[fstrim] delay fstrim timer after sonic.target

[yxieca]

Why I did it

fstrim has dependency on pmon docker.

How I did it

start fstrim timer after sonic.target.

How to verify it

local test and PR test.

Signed-off-by: Ying Xie ying.xie@microsoft.com

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106

[bingwang-ms]

The change is verified on my local build, and fstrim.timer is triggered as scheduled.

admin@vlab-01:~$ systemctl status fstrim.timer
● fstrim.timer - Discard unused blocks once a week
     Loaded: loaded (/lib/systemd/system/fstrim.timer; enabled; vendor preset: enabled)
     Active: active (waiting) since Mon 2021-09-13 08:27:06 UTC; 4min 8s ago
    Trigger: Mon 2021-09-13 08:36:44 UTC; 5min left
   Triggers: ● fstrim.service
       Docs: man:fstrim