Add some notes on raspberry pi usb + btrfs ; add some notes about arduino-cli

advanced/arduino.txt

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+Some misc notes on arduino-cli.
+
+This is mostly UNTESTED. YMMV.
+
+https://github.com/arduino/arduino-cli
+    use arm32 + multiarch for version 0.3.6-alpha on arm64 hardware
+apt install gcc-avr
+arduino-cli core update-index
+arduino-cli config init
+nano -w ~/.arduino15/arduino-cli.yaml
+board_manager:
+  additional_urls:
+    - https://raw.githubusercontent.com/adafruit/arduino-board-index/gh-pages/package_adafruit_index.json
+    - https://raw.githubusercontent.com/sparkfun/Arduino_Boards/master/IDE_Board_Manager/package_sparkfun_index.json
+    - https://dl.espressif.com/dl/package_esp32_index.json
+
+arduino-cli core update-index
+arduino-cli core install arduino:avr
+arduino-cli core install arduino:sam
+arduino-cli core install arduino:samd
+arduino-cli core install adafruit:avr
+arduino-cli core install adafruit:samd
+arduino-cli core install SparkFun:avr
+arduino-cli core install SparkFun:samd
+arduino-cli core install esp32:esp32
+arduino-cli core install esp8266:esp8266

advanced/raspberrry_pi_usb_btrfs_root.md

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+# Running a Raspberry Pi with USB based btrfs root filesystem
+
+This is a very advanced configuration and here as a set of notes for the more daring users building remixes.
+
+Please note: *you will need to change ```/dev/sda``` and ```/dev/sdb``` as well as any sizes and offsets to match your needs and approach*.
+
+``` bash
+
+dd if=/dev/sda of=/dev/sdb bs=1M
+parted /dev/sdb
+    mkpart
+    33G
+    -1
+    q
+mkfs.btrfs \
+    -L writable \
+    /dev/sdb3
+mkdir /mnt/new
+mkdir /mnt/orig
+mount /dev/sdb2 /mnt/orig
+mount -o autodefrag,compress,ssd,ssd_spread /dev/sdb3 /mnt/new
+rsync -aPr --delete-after /mnt/orig/ /mnt/new/
+nano -w /mnt/new/etc/fstab
+    btrfs filesystem type && mount options
+mount /dev/sdb1 /mnt/new/boot/firmware
+nano -w /mnt/new/boot/firmware/config.txt /mnt/new/boot/firmware/cmdline.txt
+umount /mnt/orig
+pvcreate /dev/sdb2
+vgcreate misc /dev/sdb2
+lvcreate -C y -n swap.1 -L 1G misc
+lvcreate -n scratch -L 6G misc
+mkswap /dev/mapper/misc-swap.1
+mkfs.btrfs -L scratch /dev/mapper/misc-scratch
+mkdir /scratch
+nano -w /mnt/new/etc/fstab
+    add lvm swap
+    add lvm scratch
+
+```