RPi4 setup: initial version

Added initial version of Raspberry Pi 4 setup for
developer.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>

content/reference_hardware/rpi4_setup.md

++++
+date = "2021-06-26"
+weight = 100
+
+title = "Raspberry Pi 4 Setup"
++++
+
+This is the setup recommended for developers using the Raspberry Pi 4 (RPi4) device directly.
+See [Raspberry Pi 4 Setup in LAVA]( {{< ref "/reference_hardware/rpi4_setup_lava.md" >}} )
+for setting up devices for use in LAVA.
+
+# Required items
+
+You should have at least:
+
+- 1 Raspberry Pi 4 B device
+- 1 Power adaptor
+- 1 USB to serial cable/converter (3.3v is mandatory)
+- 1 MicroSD card
+
+# RPi4 board setup
+
+## Attach serial
+
+{{% notice warning %}}
+Please ensure you are using the USB to serial which is 3.3v!
+{{% /notice %}}
+
+1. Ensure the RPi4 device is off and USB to serial cable is not attached to USB
+
+2. Connect serial to UART ports on RPi4 device.
+
+   ![RPi serial](/images/rpi-serial.jpg)
+
+   Attach cable wires to pins 6(GND), 8(TXD) and 10(RXD). Keep in mind that
+   RXD from USB serial should be connected to pin 8(TXD) on RPi4
+   and TXD from USB to pin 10(RXD).
+
+3. Connect USB serial to host.
+
+4. Connect terminal emulator on your host.
+
+   Depending on USB serial and operational system you are using the name of UART
+   may differ, as well as the terminal emulator. The parameters for your terminal
+   emulator should be `115200/8-N-1`.
+
+   For Linux-based systems it is usually `ttyUSB0` and you may use `minicom` or
+   `screen` to connect to serial:
+   ```
+   minicom -b 115200 -o -D /dev/ttyUSB0
+   ```
+   or
+   ```
+   screen /dev/ttyUSB0 115200
+   ```
+
+## Update bootrom with RPi Imager
+
+It is highly recommended to upgrade the bootrom to the latest available
+stable version!
+
+The easiest method for updating and managing the boot sequence for local usage and
+development is the [RPI Imager](https://www.raspberrypi.org/downloads) which will
+prepare the bootable SD-card with fresh updates for you:
+
+![Misc utility images](/images/rpi-imager-1-misc.png)
+
+![Select boot mode](/images/rpi-imager-2-usbboot.png)
+
+Insert the prepared SD-card into RPi4 and power on the device, on serial
+you will see the output similar to:
+```
+SIG vl805.sig 6246230ecd5b472902e6a49c95e857a5e3190c4fa6c462d6a8867e9a5e523a7c 0
+Read vl805.bin bytes    99224 hnd 0x0000009f
+PCI reset
+PCI reset
+VL805 EEPROM is up to date
+SIG pieeprom.sig 54285fd35a75f93f59e59bb04706b6a74faaeba91fad5a2cd5bb2279a8a439ab 0
+Read pieeprom.bin bytes   524288 hnd 0x00000004
+Reading EEPROM: 524288
+Bootloader EEPROM is up to date
+```
+
+{{% notice note %}}
+If you have a display attached to device, it will have a green color in case of success.
+{{% /notice %}}
+
+# Apertis OS startup
+
+The technology preview of RPi4 support has been added into Apertis in
+[v2021]( {{< ref "/release/v2021.0/releasenotes.md" >}} ) release.
+However it is recommended to use version `v2022dev3` or newer to get
+the most supported image.
+
+{{% notice note %}}
+The support for RPI Imager is ongoing.
+{{% /notice %}}
+
+1. On your host [download the minimal image](https://images.apertis.org/daily/v2022dev3/) for
+   Raspberry Pi 4 `arm64` architecture and corresponding `bmap` file (both must have
+   `_rpi64_` suffix in file names). For instance,
+   for `apertis_v2022dev3-minimal-arm64-rpi64_20210630.0018.img.gz`:
+
+   ```
+   wget https://images.apertis.org/daily/v2022dev3/20210630.0018/arm64/minimal/apertis_v2022dev3-minimal-arm64-rpi64_20210630.0018.img.gz
+   wget https://images.apertis.org/daily/v2022dev3/20210630.0018/arm64/minimal/apertis_v2022dev3-minimal-arm64-rpi64_20210630.0018.img.bmap
+   ```
+
+2. Flash the downloaded images onto SD-card with `bmaptool`, assuming
+   the `/dev/mmcblk0` is your card reader device:
+   ```
+   sudo bmaptool copy apertis_v2022dev3-minimal-arm64-rpi64_20210630.0018.img.gz /dev/mmcblk0
+   ```
+
+   {{% notice warning %}}
+   Ensure that the target device is not your hard disk to avoid damaging your data!
+   {{% /notice %}}
+
+3. Insert the prepared SD-card into RPi4 device and power on the board.
+   In a few seconds you will see on serial the U-Boot is starting:
+   ```
+   U-Boot 2021.01+dfsg-3apertis2bv2022dev3b1 (Jun 22 2021 - 20:50:32 +0000)
+
+   DRAM:  1.9 GiB
+   RPI 4 Model B (0xb03111)
+   ...
+   ```
+4. The Apertis will ask the login prompt:
+   ```
+   Apertis v2022dev3 apertis ttyS1
+   apertis login:
+   ```
+   You can login to the console with the username `user` and password `user`.