A script that asks to make the root filesystem read-writable for subsequent changes and additions by the user.

rw-rootfs

#!/bin/sh -e 
##!! PLEASE USE THIS SCRIPT WITH CAUTION - AND AT YOUR OWN RISK          !!##
##!! IT HAS BEEN KNOWN TO CAUSE RESETS AND WIPE DATA ON SOME CHROMEBOXES !!##

APPLICATION="${0##*/}"
ANSWER=''
CURRENTROOT=''
CURRENTKERNEL=''
ROOTDEV=''
ROOTDEVPFX=''

## Exits the script with exit code $1, spitting out message $@ to stderr
error() {
    local ecode="$1"
    shift
    echo "$*" 1>&2
    exit "$ecode"
}

## Report dev_boot_legacy and dev_boot_usb flags
## Check and set dev_boot_signed_only flag if needed.
checkflags() {
    echo -n "## "
    sudo crossystem dev_boot_usb dev_boot_legacy dev_boot_signed_only
    echo " ##"
    boot="$(sudo crossystem dev_boot_usb dev_boot_legacy dev_boot_signed_only)"
    # db_usb and db_legacy can be off, db_signed_only should be off.
    echo "$boot" | {
        read -r usb legacy signed
        suggest=''
        if [ "$usb" = 1 ]; then
            echo "NOTE: USB booting <Ctrl+U> is enabled." 1>&2
        else
            echo "WARNING: USB booting is disabled." 1>&2
            suggest="$suggest dev_boot_usb=1"
        fi
        if [ "$legacy" = 1 ]; then
            echo "NOTE: Legacy booting <Ctrl+L> is enabled." 1>&2
        else
            echo "WARNING: Legacy booting is disabled." 1>&2
            suggest="$suggest dev_boot_legacy=1"
        fi
        if [ -n "$suggest" ]; then
            echo "To enable, you can use the following command: sudo crossystem$suggest" 1>&2
            sleep 3
        fi
        if [ "$signed" = 1 ]; then
            # Only disable signed booting if the user hasn't to ensure booting unverified kernels
            echo "WARNING: Signed boot verification is enabled; disabling it to ensure booting unverified kernel." 1>&2
            echo "You can enable it again using: sudo crossystem dev_boot_signed_only=1" 1>&2
            sudo crossystem dev_boot_signed_only=0 || true
            sleep 3
        else
            echo "NOTE: Signed boot verification is disabled, you're good to go..." 1>&2
        fi
        sleep 2
    } 
}

#Following routine borrowed from @drinkcat ;)
ROOTDEV="$(rootdev -d -s)"
if [ -z "$ROOTDEV" ]; then
    error 1 "Cannot find root device."
fi
if [ ! -b "$ROOTDEV" ]; then
    error 1 "$ROOTDEV is not a block device."
fi
# If $ROOTDEV ends with a number (e.g. mmcblk0), partitions are named
# ${ROOTDEV}pX (e.g. mmcblk0p1). If not (e.g. sda), they are named
# ${ROOTDEV}X (e.g. sda1).
ROOTDEVPFX="$ROOTDEV"
if [ "${ROOTDEV%[0-9]}" != "$ROOTDEV" ]; then
    ROOTDEVPFX="${ROOTDEV}p"
fi

CURRENTROOT="$(rootdev -s)"
if [ $CURRENTROOT = ${ROOTDEVPFX}3 ]; then CURRENTKERNEL=2; else CURRENTKERNEL=4; fi

USAGE="
$APPLICATION [no options]

### A script that asks the user to make the root filesystem (${CURRENTROOT})
##+ read-writable for subsequent changes and additions by the user. 
"

if [ $# -gt 0 ]; then error 0 "$USAGE"; fi

if sudo mount -i -o remount,rw / 2>/dev/null; then
  echo "Your rootfs (${CURRENTROOT}) is already mounted read-write ..."
  echo "*** $(mount | grep ' / ') ***"; exit 0
fi

echo -n "Perform  REMOVAL of rootfs verification on partition '${CURRENTKERNEL}' (Y/n/q) ? " 1>&2
read ANSWER; if [ -z "$ANSWER" ]; then ANSWER='y'; fi

case $ANSWER in
  [yY]*) checkflags 
         echo
         echo "sudo /usr/share/vboot/bin/make_dev_ssd.sh --remove_rootfs_verification --partitions $CURRENTKERNEL" 1>&2
               sudo /usr/share/vboot/bin/make_dev_ssd.sh --remove_rootfs_verification --partitions $CURRENTKERNEL || ret=$?
         if [ $ret -gt 0 ]; then
           echo "Removal of rootfs verification failed, now we'll try using the '--force' option..."
           echo "sudo /usr/share/vboot/bin/make_dev_ssd.sh --force --remove_rootfs_verification --partitions $CURRENTKERNEL"
                 sudo /usr/share/vboot/bin/make_dev_ssd.sh --force --remove_rootfs_verification --partitions $CURRENTKERNEL || ret=$?
           if [ "$ret" -gt 0 ]; then
             error 2 "Sorry but REMOVAL of rootfs verification failed on partition ${CURRENTROOT}/${CURRENTKERNEL}."
           fi
         fi
         echo "*** Rebooting in 10 seconds to make changes effective ***" 1>&2
         echo -n "... Press Ctrl-C to ABORT ... " 1>&2
         sleep 10 && sudo reboot && exit $ret
      ;;
  [nN]*) error 0 "Skipping REMOVAL of rootfs verification on partition '$CURRENTKERNEL' for now..."
      ;;
  [qQ]*) error 0 "Quitting - no changes made..."
      ;;
  *)     error 1 "Not a valid choice, exiting..."
      ;;
esac