udhcpc: support resolv.conf symlinks

Often it is desirable to have /etc/ be on read-only storage (well, the
whole rootfs) but have things like /etc/resolv.conf be symlinks to a
writable location.  Tweak the simple script to support that.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

examples/udhcp/simple.script

@@ -34,13 +34,17 @@ case "$1" in
 		fi
 
 		echo "Recreating $RESOLV_CONF"
-		echo -n > $RESOLV_CONF-$$
-		[ -n "$domain" ] && echo "search $domain" >> $RESOLV_CONF-$$
+		# If the file is a symlink somewhere (like /etc/resolv.conf
+		# pointing to /run/resolv.conf), make sure things work.
+		realconf=$(realpath "$RESOLV_CONF" 2>/dev/null || echo "$RESOLV_CONF")
+		tmpfile="$realconf-$$"
+		> "$tmpfile"
+		[ -n "$domain" ] && echo "search $domain" >> "$tmpfile"
 		for i in $dns ; do
 			echo " Adding DNS server $i"
-			echo "nameserver $i" >> $RESOLV_CONF-$$
+			echo "nameserver $i" >> "$tmpfile"
 		done
-		mv $RESOLV_CONF-$$ $RESOLV_CONF
+		mv "$tmpfile" "$realconf"
 		;;
 esac