make detach_img idempotent

imgtool

@@ -99,8 +99,20 @@ attach_img() {
     fi
 }
 
+is_mounted() {
+    local mountpoint="$1"
+    local mountpoint_abs="$(cd "${mountpoint}" && pwd)"
+    local tmp=$(mount | awk "\$1 == \"${mountpoint_abs}\"")
+    if [ -z "${tmp}" ]; then
+        return 1
+    else
+        return 0
+    fi
+}
+
 detach_img() {
     local mountpoint="$1"
+    is_mounted "${mountpoint}" || return 0
     case ${OS} in
 	Darwin)
 	    sudo hdiutil detach "${mountpoint}"