Move back isset to the functions-common

isset function was moved to config file related functions by accident,
this change also simplfies the isset in a bash >=4.2 way.

All supported distro has at least bash 4.2. (RHEL6 used 4.1)

Change-Id: Id644b46ff9cdbe18cde46e96aa72764e1c8653ac
diff --git a/functions-common b/functions-common
index 48e400d..56fa64a 100644
--- a/functions-common
+++ b/functions-common
@@ -62,6 +62,9 @@
     $xtrace
 }
 
+function isset {
+    [[ -v "$1" ]]
+}
 
 # Control Functions
 # =================