[reboot-cause] Porting PR to fix a broken symlink of previous-reboot-cause file removal issue (sonic-host-services #46)

[mlok-nokia]

Why I did it

Porting/cherry-pick PR sonic-net/sonic-host-services#46
"show reboot-cause history" shows empty history. When the previous-reboot-cause has a broken symlink, And rebooting the system will not be able to generate a new symlink of the new previous-reboot-cause.

admin@sonic:~$ show reboot-cause history 
Name    Cause    Time    User    Comment
------  -------  ------  ------  ---------

How I did it

Somehow, when the symlink file /host/reboot-cause/previous-reboot-cause is broken (which its destination files doesn't exist in this case), the current condition check "if os.path,exists(PREVIOUS_REBOOT_CAUSE_FILE)" will return False in determine-reboot-cause script. Hence, the current previous-reboot-cause is not been removed and the recreation of the new previous-reboot-cause failed. In case of previous-reboot-cause is a broken synlink file, add condition os.path.islink(PREVIOUS_REBOOT_CAUSE) to check and allow the remove operation happens.

How to verify it

1. Manually make the /host/reboot-cause/previous-reboot-cause to be a broken symlink file by removing its destination file
2. reboot the system. "show reboot-cause history" should show the correct info

admin@ixre-egl-board25:~$ show reboot-cause history 
Name                 Cause                                    Time                             User     Comment
-------------------  ---------------------------------------  -------------------------------  -------  ----------
2023_03_01_17_41_31  reboot                                   Wed 01 Mar 2023 05:39:27 PM UTC  admin    N/A
2023_03_01_17_14_47  reboot                                   Wed 01 Mar 2023 05:12:05 PM UTC  admin    N/A
2023_03_01_17_14_18  reboot                                   Wed 01 Mar 2023 05:12:05 PM UTC  admin    N/A
2023_03_01_16_20_52  reboot                                   Wed 01 Mar 2023 04:18:22 PM UTC  admin    N/A
2023_03_01_16_20_27  reboot                                   Wed 01 Mar 2023 04:18:22 PM UTC  admin    N/A

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

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202205
• 202211

Description for the changelog

Ensure to add label/tag for the feature raised. example - PR#2174 under sonic-utilities repo. where, Generic Config and Update feature has been labelled as GCU.

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)

[mlok-nokia]

@abdosi and @judyjoseph , Per request, this PR is cherry-pick/porting from the master branch sonic-net/sonic-host-services#46. Please review it.

[prgeor]

@mlok-nokia when the issue is hit, there are no reboot-cause json files in /host/reboot-cause/history/ ?

[mlok-nokia]

@mlok-nokia when the issue is hit, there are no reboot-cause json files in /host/reboot-cause/history/ ?

Correct. When the issue happens. File /host/reboot-cause/previous-reboot-cause reference to a file in /host/reboot-cause/history/ which doesn't exist. This PR is to correct the problem which already happened and allow determine-reboot-cause to run success.