How to make an SBC appear as USB storage device and have access to the files via Samba

auto-switch.sh

#!/bin/bash

while :
do
  sleep 1
  if [ -e /mnt/write ]
  then
    if grep -q usb /sys/kernel/config/usb_gadget/g1/UDC
    then
      sync
      echo '' > /sys/kernel/config/usb_gadget/g1/UDC
      sync
      umount /mnt/usb
      fsck -y /dev/loop0p1
      mount -o rw /mnt/usb
    fi
  else
    if ! grep -q . /sys/kernel/config/usb_gadget/g1/UDC
    then
      sync
      mount -o remount,ro /mnt/usb
      sync
      echo musb-hdrc.1.auto > /sys/kernel/config/usb_gadget/g1/UDC
    fi
  fi
done

rc.local

#!/bin/sh

modprobe libcomposite
modprobe usb_f_mass_storage
mkdir /sys/kernel/config/usb_gadget/g1
cd /sys/kernel/config/usb_gadget/g1
echo 0xabcd > idVendor
echo 0x1234 > idProduct
mkdir strings/0x409
echo myserial > strings/0x409/serialnumber
echo mymfg > strings/0x409/manufacturer
echo myproduct > strings/0x409/product
mkdir configs/c.1
echo 120 > configs/c.1/MaxPower
mkdir functions/mass_storage.0
echo /dev/mmcblk0p3 > functions/mass_storage.0/lun.0/file
ln -s functions/mass_storage.0 configs/c.1
echo musb-hdrc.1.auto > UDC
cd

losetup --partscan /dev/loop0 /dev/mmcblk0p3
mount -o ro /mnt/usb
mount -o remount,sync /
echo 100 > /proc/sys/vm/dirty_expire_centisecs

bash -c 'cd /sys/kernel/config/usb_gadget/g1/functions/mass_storage.0/lun.0/ ; while :; do sleep 1 ; grep -q /dev/mmcblk0p3 file || echo /dev/mmcblk0p3 > file ; echo 1 > /proc/sys/vm/drop_caches ; done &' &
bash -c '/root/auto-switch.sh &' &