Introduce the asic_subtype field for adding the sub platform variants.

[judyjoseph]

Why I did it

Introduce the asic_subtype field for adding the sub platform variants.
This is to add broadcom-dnx as a asic subtype of broadcom. For the broadcom XGS type ASIC platforms the asic_subtype will be "broadcom" itsef.

So for DNX platforms
"asic_type": "broadcom",
"asic_subtype": "broadcom-dnx",

XGS platforms
"asic_type": "broadcom",
"asic_subtype": "broadcom",

Related PR : sonic-net/sonic-swss#2178

How I did it

The TARGET_MACHINE variable will contain $($_MACHINE), $($_DEPENDENT_MACHINE) which will have values like broadcom, broadcom-dnx for the different asic types within the $(sonic_asic_platform). This will be used to populate the asic_subtype in sonic_version.yml and inturn into swss_vars

The asic_type will come from the PLATFORM which we use while compilation eg: broadcom. The asic_subtypes will be various $($_MACHINE), $($_DEPENDENT_MACHINE) variables used to compile and get the asic family specific images.

How to verify it

Verified that "sub_platform" environment variable gets populated, when used along with sonic-net/sonic-swss#2178.

root@str-s6000-acs-8:/# cat /etc/sonic/sonic_version.yml 
---
build_version: 'master.0-dirty-20220319.025049'
debian_version: '11.2'
kernel_version: '5.10.0-8-2-amd64'
asic_type: broadcom
asic_subtype: 'broadcom'
commit_id: 'fb7f04614'
branch: 'master'
release: 'none'
build_date: Sat Mar 19 14:03:52 UTC 2022

root@str-s6000-acs-8:/# cat /usr/share/sonic/templates/swss_vars.j2 
{
    "asic_type": "{{ asic_type }}",
    {% if asic_subtype is defined and asic_subtype != '' -%}
    "asic_subtype": "{{ asic_subtype }}",
    {% endif -%}
    "asic_id": "{{ DEVICE_METADATA.localhost.asic_id }}",
    "mac": "{{ DEVICE_METADATA.localhost.mac }}",
    "resource_type": "{{ DEVICE_METADATA.localhost.resource_type }}",
    "synchronous_mode": {% if DEVICE_METADATA.localhost.synchronous_mode == "disable" %}"disable"{% else %}"enable"{% endif %}
}

admin@str-s6000-acs-8:~$ docker exec -it swss bash
root@str-s6000-acs-8:/# sonic-cfggen -d -y /etc/sonic/sonic_version.yml -t /usr/share/sonic/templates/swss_vars.j2 
{
    "asic_type": "broadcom",
    "asic_subtype": "broadcom",
    "asic_id": "",
    "mac": "ec:f4:bb:fe:80:84",
    "resource_type": "",
    "synchronous_mode": "enable"}

In case of dnx-based devices 

admin@sonic:~$ cat /etc/sonic/sonic_version.yml 
---
build_version: 'master.0-dirty-20220319.025049'
debian_version: '11.2'
kernel_version: '5.10.0-8-2-amd64'
asic_type: broadcom
asic_subtype: 'broadcom-dnx'
commit_id: 'fb7f04614'
branch: 'master'
release: 'none'
build_date: Sat Mar 19 14:19:24 UTC 2022

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

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111

Description for the changelog

Link to config_db schema for YANG module changes

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

[prsunny]

lgtm