add parameter for checking out a specific git branch

scripts/common.sh

@@ -12,10 +12,16 @@ function get_sources() {
 
 # $1: source package name
 # $2: git repository
+# $3: (optional) specifc branch or tag
 function get_git_sources() {
 	git clone $2 $1
 	cd $1
-	git tag --sort "version:refname" -l | tail -n1 | xargs git checkout
+	if [[ ! -z $3 ]] # checks if $3 is not empty
+	then 
+		git checkout $3
+	else
+		git tag --sort "version:refname" -l | tail -n1 | xargs git checkout
+	fi
 }
 
 # $1: patch directory name