Check if symlinks are valid

When using root option and $initd/$bn is a symlink, the script would fail because
the symlink points to a path on target. For example:

/path/to/target/rootfs/etc/init.d/syslog -> /etc/init.d/syslog.busybox

Hence, [ -f /path/to/target/rootfs/etc/init.d/syslog ] condition would return
false.

This patch adds the posibility to check whether the file the symlink points to
actually exists in rootfs path and then continue.

Upstream-Status: Pending

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com> 

Index: git/update-rc.d
===================================================================
--- git.orig/update-rc.d	2013-01-16 12:12:58.349814356 +0200
+++ git/update-rc.d	2013-01-16 13:02:42.490864939 +0200
@@ -147,13 +147,29 @@
 bn=$1
 shift
 
+sn=$initd/$bn
+if [ -L "$sn" -a -n "$root" ]; then
+	readlink=$(which readlink)
+
+	if [ -n "$readlink" ]; then
+		sn=$($readlink "$sn")
+		case "$sn" in
+			/*) sn=${root}${sn} ;;
+			*)  sn=$initd/$sn ;;
+		esac
+	else
+		echo "update-rc.d: readlink tool not present, cannot check whether \
+				$sn symlink points to a valid file." >&2
+	fi
+fi
+
 if [ $1 != "remove" ]; then
-	if [ ! -f "$initd/$bn" ]; then
+	if [ ! -f "$sn" ]; then
 		echo "update-rc.d: $initd/$bn: file does not exist" >&2
 		exit 1
 	fi
 else
-	if [ -f "$initd/$bn" ]; then
+	if [ -f "$sn" ]; then
 		if [ $force -eq 1 ]; then
 			echo "update-rc.d: $initd/$bn exists during rc.d purge (continuing)" >&2
 		else