overlay: Lift add implementation to overlay_add()

Isolate the functionality to improve readability

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: I1f0abdd33291aa14f53c0220fb5c714b181acb97
diff --git a/overlay/overlay b/overlay/overlay
index 2400b9f..f01f8b4 100755
--- a/overlay/overlay
+++ b/overlay/overlay
@@ -14,6 +14,13 @@
     cut -d= -f2
 }
 
+overlay_add() {
+    o_lowerdir="$1"
+    o_upperdir=$(mktemp -d)
+    o_workdir=$(mktemp -d)
+    mount -t overlay -o lowerdir="${o_lowerdir}",upperdir=${o_upperdir},workdir=${o_workdir} overlay "${o_lowerdir}"
+}
+
 if [ $# -lt 2 ]
 then
     cat $0
@@ -22,10 +29,7 @@
 
 case $1 in
     add)
-        o_lowerdir="$2"
-        o_upperdir=$(mktemp -d)
-        o_workdir=$(mktemp -d)
-        mount -t overlay -o lowerdir="${o_lowerdir}",upperdir=${o_upperdir},workdir=${o_workdir} overlay "${o_lowerdir}"
+        overlay_add $2
         ;;
     remove)
         o_lowerdir="$2"