add a --root option so we can generate with DESTDIR installs

--- a/image/usr/sbin/update-ca-certificates
+++ b/image/usr/sbin/update-ca-certificates
@@ -30,6 +30,8 @@ LOCALCERTSDIR=/usr/local/share/ca-certificates
 CERTBUNDLE=ca-certificates.crt
 ETCCERTSDIR=/etc/ssl/certs
 HOOKSDIR=/etc/ca-certificates/update.d
+ROOT=""
+RELPATH=""
 
 while [ $# -gt 0 ];
 do
@@ -59,13 +61,25 @@ do
     --hooksdir)
       shift
       HOOKSDIR="$1";;
+    --root|-r)
+      shift
+      # Needed as c_rehash wants to read the files directly.
+      # This gets us from $CERTSCONF to $CERTSDIR.
+      RELPATH="../../.."
+      ROOT=$(readlink -f "$1");;
     --help|-h|*)
-      echo "$0: [--verbose] [--fresh]"
+      echo "$0: [--verbose] [--fresh] [--root <dir>]"
       exit;;
   esac
   shift
 done
 
+CERTSCONF="$ROOT$CERTSCONF"
+CERTSDIR="$ROOT$CERTSDIR"
+LOCALCERTSDIR="$ROOT$LOCALCERTSDIR"
+ETCCERTSDIR="$ROOT$ETCCERTSDIR"
+HOOKSDIR="$ROOT$HOOKSDIR"
+
 if [ ! -s "$CERTSCONF" ]
 then
   fresh=1
@@ -94,7 +107,7 @@ add() {
                                                   -e 's/,/_/g').pem"
   if ! test -e "$PEM" || [ "$(readlink "$PEM")" != "$CERT" ]
   then
-    ln -sf "$CERT" "$PEM"
+    ln -sf "${RELPATH}${CERT#$ROOT}" "$PEM"
     echo "+$PEM" >> "$ADDED"
   fi
   # Add trailing newline to certificate, if it is missing (#635570)