Upgrade the root partition mount to r/w before trying to write to it.

MFC after: 3 days

release/picobsd/mfs_tree/etc/rc

@@ -1,4 +1,5 @@
 #!/bin/sh
+# $FreeBSD$
 
 stty status '^T'
 trap : 2
@@ -10,6 +11,7 @@ dev=`sysctl -n machdep.guessed_bootdev`
 [ -c "${dev}" ] || dev="/dev/fd0"
 
 trap "echo 'Reboot interrupted'; exit 1" 3
+set `df /`; mount -u $8 / # upgrade mount to rw
 echo "Loading /etc from MFS:/fd ..."
 cp -Rp /fd/* /
 echo "Updating /etc from ${dev}..."