Move Chipyard CI to Github Actions

- As similar as possible to the circle ci code.
- The `.github/README.md` file has a fair amount of documentation for this.
- Creates a worfklow file
  - re-uses most of the circleci/scipts unchanged
  - defines a number of *Composite Actions* which are like YML subroutines
- Removes the circle-ci code
- Points the CI badge in the top level README to use the GA test result

.github/scripts/clean-old-files.sh

@@ -0,0 +1,29 @@
+#!/bin/bash
+
+# clean directories that are older than 14 days
+# argument is used as the directory to look in
+
+age () {
+	local AGE_SEC
+	local CUR_SEC
+	local DIFF_SEC
+	local SEC_PER_DAY
+
+	SEC_PER_DAY=86400
+
+	CUR_SEC=$(date +%s)
+	AGE_SEC=$(stat -c %Y -- "$1")
+	DIFF_SEC=$(expr $CUR_SEC - $AGE_SEC)
+
+	echo $(expr $DIFF_SEC / $SEC_PER_DAY)
+}
+
+for d in $1/*/ ; do
+	DIR_AGE="$(age $d)"
+	if [ $DIR_AGE -ge 14 ]; then
+		echo "Deleting $d since is it $DIR_AGE old"
+		rm -rf $d
+	else
+	    echo "Keep $d since it is $DIR_AGE old"
+	fi
+done