[sfputil] Add support of platform.json (sonic-net#767)

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

> This PR is dependent on [sonic-platform-common/pull/72](sonic-net/sonic-platform-common#72)

All three PRs are necessary to run `show interfaces transceiver` command.

1. [sonic-net/sonic-buildimage#3912](sonic-net/sonic-buildimage#3912)
2. [sonic-net/sonic-platform-common#72](sonic-net/sonic-platform-common#72)
3. [sonic-net#767](sonic-net#767)


**- What I did**
Add support of platform.json in sfputil to get correct output of `show interfaces transceiver`


**- How to verify it**
Check whether all the below-mentioned CLI's are working correctly.
```
Usage: show interfaces transceiver [OPTIONS] COMMAND [ARGS]...

  Show SFP Transceiver information

Options:
  -?, -h, --help  Show this message and exit.

Commands:
  eeprom    Show interface transceiver EEPROM information
  lpmode    Show interface transceiver low-power mode status
  presence  Show interface transceiver presence
```

sfputil/main.py

@@ -30,7 +30,8 @@
 
 # Global platform-specific sfputil class instance
 platform_sfputil = None
-
+PLATFORM_JSON = 'platform.json'
+PORT_CONFIG_INI = 'port_config.ini'
 
 # ========================== Syslog wrappers ==========================
 
@@ -325,14 +326,13 @@ def get_path_to_port_config_file():
     hwsku_path = "/".join([platform_path, hwsku])
 
     # First check for the presence of the new 'port_config.ini' file
-    port_config_file_path = "/".join([hwsku_path, "port_config.ini"])
+    port_config_file_path = "/".join([platform_path, PLATFORM_JSON])
     if not os.path.isfile(port_config_file_path):
-        # port_config.ini doesn't exist. Try loading the legacy 'portmap.ini' file
-        port_config_file_path = "/".join([hwsku_path, "portmap.ini"])
+        # platform.json doesn't exist. Try loading the legacy 'port_config.ini' file
+        port_config_file_path = "/".join([hwsku_path, PORT_CONFIG_INI])
 
     return port_config_file_path
 
-
 # Loads platform specific sfputil module from source
 def load_platform_sfputil():
     global platform_sfputil