[portconfig] Fallback to port_config.ini if hwsku.json is not available (sonic-net#5434)

**- Why I did it**
As discussed, This PR covers the below points.
1. Give precedence to platform.json only if both platform.json and  hwsku.json file exist.  In case only platform.json exists, we don’t allow breakout for that HWSKU and fallback to port_config.ini.
 
**- How I did it**
check for `hwsku.json` file presence under get_path_to_port_config_file function.

Signed-off-by: Sangita Maity <[email protected]>

Author: samaity

src/sonic-py-common/sonic_py_common/device_info.py

@@ -20,6 +20,9 @@
 PORT_CONFIG_FILE = "port_config.ini"
 PLATFORM_JSON_FILE = "platform.json"
 
+# HwSKU configuration file name
+HWSKU_JSON_FILE = 'hwsku.json'
+
 # Multi-NPU constants
 # TODO: Move Multi-ASIC-related functions and constants to a "multi_asic.py" module
 NPU_NAME_PREFIX = "asic"
@@ -247,8 +250,13 @@ def get_path_to_port_config_file(hwsku=None, asic=None):
 
     port_config_candidates = []
 
-    # Check for 'platform.json' file presence first
-    port_config_candidates.append(os.path.join(platform_path, PLATFORM_JSON_FILE))
+    # Check for 'hwsku.json' file presence first
+    hwsku_json_file = os.path.join(hwsku_path, HWSKU_JSON_FILE)
+
+    # if 'hwsku.json' file is available, Check for 'platform.json' file presence,
+    # if 'platform.json' is available, APPEND it. Otherwise, SKIP it.
+    if os.path.isfile(hwsku_json_file):
+        port_config_candidates.append(os.path.join(platform_path, PLATFORM_JSON_FILE))
 
     # Check for 'port_config.ini' file presence in a few locations
     if asic: