[202012] [Celestica/Seastone2] Install sonic_platform

[bluecmd]

Why I did it

This is part of fixing #8212.
This changes fixes installation of sonic_platform to be installed for both python2 and python3.
Right now neither is installed.

How I did it

Looked at the error messages when booting a clean Seastone2 as well as the packaging files.

How to verify it

Verify that sonic_platform is installed.

(vrf:mgmt)bluecmd@ixp-kg-sw1:~$ python2 -c 'import sonic_platform'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ImportError: No module named sonic_platform
(vrf:mgmt)bluecmd@ixp-kg-sw1:~$ python3 -c 'import sonic_platform'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ModuleNotFoundError: No module named 'sonic_platform'

After:

(vrf:mgmt)bluecmd@ixp-kg-sw1:~$ python2 -c 'import sonic_platform'
(vrf:mgmt)bluecmd@ixp-kg-sw1:~$ python3 -c 'import sonic_platform'

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

• 201811
• 201911
• 202006
• 202012
• 202106

Description for the changelog

cel/seastone2: install sonic_platform

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

[bluecmd]

@sujinmkang can you maybe review this?

[sujinmkang]

@bluecmd Can you please rebase this PR again?

[bluecmd]

@sujinmkang Rebase was done :-)

[bluecmd]

@lguohan OK to merge?