Add Codecov badge and remove report style (#8896)

.codecov.yml

@@ -1,5 +1,5 @@
 comment:
-  layout: "diff, flags, files"
+  layout: "flags, files"
   behavior: default
   require_changes: true   # only comment on changes in coverage
   require_base: yes       # [yes :: must have a base report to post]

README.md

@@ -4,6 +4,7 @@
 [![Windows Build Status][appveyor-badge]][appveyor]
 [![GoDoc][godoc-badge]][godoc]
 [![GoReportCard][report-badge]][report]
+[![codecov](https://codecov.io/gh/hashicorp/packer/branch/master/graph/badge.svg)](https://codecov.io/gh/hashicorp/packer)
 
 [travis-badge]: https://travis-ci.org/hashicorp/packer.svg?branch=master
 [travis]: https://travis-ci.org/hashicorp/packer