[syseeprom] Enable display of vendor extension TLV content (sonic-net…

…#270)

Description
The code that decodes the content of the ONIE syseeprom includes a flag to
enable/disable displaying the content of the vendor extension TLV. This flag is
currently set to 'disable'. Hence the 'show platform syseeprom' command shows
the presence and size of the vendor extension TLV but does not show its
content. This commit sets the flag to 'enable' so that the vendor extension TLV
content is displayed.
Motivation and Context
The 'show platform syseeprom' command shows that the Vendor Extension TLV is present but does not show its content.
Here's what that looks like on an example platform.

sonic_platform_base/sonic_eeprom/eeprom_tlvinfo.py

@@ -68,7 +68,7 @@ class TlvInfoDecoder(eeprom_base.EepromDecoder):
     _TLV_CODE_QUANTA_MODEL_NAME = _TLV_CODE_UNDEFINED
 
     # TLV Value Display Switch
-    _TLV_DISPLAY_VENDOR_EXT     = False
+    _TLV_DISPLAY_VENDOR_EXT     = True
 
 
     def __init__(self, path, start, status, ro, max_len=_TLV_INFO_MAX_LEN):