CVL Changes #1: Initial CVL code reorganization and common infra chan…

…ges (#18)

- Why I did it

As part of CVL code update and new features addition, the existing CVL code has to be reorganized and common infra needs to be changed.

- How I did it

N/A.
Please refer to change log for more details.

- How to verify it

No specific test cases to be executed. Run cvl go test cases using - 'make cvl-test'.

- Description for the changelog

Modify Makefiles
Modify CVL Makefile for cleanall rule
Modify Makefile for CVL YIN schema generation
Modify Makefile for CVL test YIN schema generation
Moved cache, syntax and semantic related code from cvl.go to cvl_cache.go, cvl_syntax.go and cvl_semantics.go respectively .
Added new API signatures for integration. Implementation will be added in subsequent PRs.
Copied sonic-port.yang and sonic-acl.yang to CVL own's test folder.

cvl/Makefile

@@ -70,3 +70,5 @@ clean:
 	$(RM) -r $(CVL_PKG)
 	$(RM) -r $(CVL_TEST_DIR)
 
+cleanall:clean
+