Add support for custom check in header checker (FreeRTOS#1285)

This allows derived classes to override customCheck method to inject
additional checks for some files. This is currently needed to accept
ARM copyright in ARMv8-M files - FreeRTOS#1147.

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>

Author: aggarg

.github/scripts/common/header_checker.py

@@ -216,6 +216,10 @@ def isValidFile(self, path):
                 print("PASS")
                 print("-" * 85)
                 return True
+            elif self.customCheck(path):
+                print("PASS")
+                print("-" * 85)
+                return True
             elif self.isThirdPartyFile(path):
                 print("FAIL")
                 print("-" * 85)
@@ -435,3 +439,6 @@ def validateSpdxLine(self, lines):
             else:
                 error_count += 1
         return error_count == 0
+
+    def customCheck(self, path):
+        return False