[decode-syseeprom] Fix setting use_db based on support_eeprom_db (#2270)

What I did
Set use_db after support_eeprom_db is determined. The issue was affecting platform Arista platforms, where db is not used for syseeprom.

How to verify it
Verified on dut by running "show platform syseeprom" and "decode-syseeprom -d"

scripts/decode-syseeprom

@@ -228,8 +228,6 @@ def main():
 
     (opts, args) = get_cmdline_opts()
 
-    use_db = opts.db and support_eeprom_db
-
     # Get platform name
     platform = device_info.get_platform()
 
@@ -238,6 +236,8 @@ def main():
     if any(re.match(p, platform) for p in platforms_without_eeprom_db):
         support_eeprom_db = False
 
+    use_db = opts.db and support_eeprom_db
+
     if opts.mgmtmac:
         print_mgmt_mac(use_db)
     elif opts.serial:

tests/decode_syseeprom_test.py

@@ -192,3 +192,12 @@ def test_print_model(self, capsys):
         decode_syseeprom.print_model(True)
         captured = capsys.readouterr()
         assert captured.out == 'S6100-ON\n'
+
+    @mock.patch('os.geteuid', lambda: 0)
+    @mock.patch('sonic_py_common.device_info.get_platform', lambda: 'arista')
+    @mock.patch('decode-syseeprom.read_and_print_eeprom')
+    @mock.patch('decode-syseeprom.read_eeprom_from_db')
+    def test_support_platforms_not_db_based(self, mockDbBased, mockNotDbBased):
+        decode_syseeprom.main()
+        assert mockNotDbBased.called
+        assert not mockDbBased.called