RPi4 LAVA setup: added initial version

Added description how to setup the RPi 4 board to use
network boot for LAVA.

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

content/reference_hardware/rpi4_setup_lava.md

++++
+date = "2021-06-26"
+weight = 100
+
+title = "Raspberry Pi 4 Setup in LAVA"
++++
+
+These are the instructions to prepare a device for use as a DUT in the
+Apertis LAVA automated test laboratory. See
+[Reference_Hardware/rpi4_setup]( {{< ref "/reference_hardware/rpi4_setup.md" >}} )
+for the recommended setup for developers' devices.
+
+# Power control
+
+It is possible to use 5V PSU which can provide at least 2.5A to power the RPi4.
+According the [HAT design guide](https://github.com/raspberrypi/hats/blob/master/designguide.md#back-powering-the-pi-via-the-gpio-header):
+
+> it is possible to power the Pi by supplying 5V through the GPIO header
+> pins 2,4 and GND. The acceptable input voltage range is 5V ±5%.
+
+See the GPIO header pinout at [pinout.xyz](https://pinout.xyz/).
+
+# Boot control
+
+The Raspberry Pi 4 OTP first stage bootloader is using a second stage located in the EEPROM chip.
+
+Depending on the [boot mode](https://www.raspberrypi.org/documentation/hardware/raspberrypi/bootmodes/bootflow_2711.md)
+flashed into SPI EEPROM the second stage bootloader is able to boot the DUT
+from SD-card, USB mass storage device or network.
+
+# Flash EEPROM for network boot
+
+Do the initial setup as described in [rpi4_setup]( {{< ref "/reference_hardware/rpi4_setup.md" >}} ).
+
+Low level tuning of bootrom is possible with the tools from `rpi-eeprom` package.
+A distribution which ships the `rpi-eeprom` package is needed, for instance NOOBS, Raspbian or Ubuntu.
+Boot the RPi4 device with a selected distro and proceed in terminal.
+
+Check the list of [available stable bootroms](https://github.com/raspberrypi/rpi-eeprom/tree/master/firmware/stable)
+and export variable with the last available version, for example:
+```
+PI_EEPROM_VERSION=2021-04-29
+```
+
+Download the bootrom:
+```
+wget https://github.com/raspberrypi/rpi-eeprom/raw/master/firmware/stable/pieeprom-${PI_EEPROM_VERSION}.bin
+```
+
+Extract and save the default config:
+```
+rpi-eeprom-config pieeprom-${PI_EEPROM_VERSION}.bin > bootconf.txt
+```
+
+Change the bootrom configration file `bootconf.txt` with any editor according
+[documentation](https://www.raspberrypi.org/documentation/hardware/raspberrypi/bcm2711_bootloader_config.md).
+
+The configuration below is forcing the board to use network boot only and use `bcm2711-rpi-4-b`
+directory on server side to search boot files:
+```
+[all]
+BOOT_UART=1
+WAKE_ON_GPIO=1
+POWER_OFF_ON_HALT=0
+BOOT_ORDER=0xf2
+TFTP_PREFIX=1
+TFTP_PREFIX_STR=bcm2711-rpi-4-b/
+```
+
+Generate the bootrom with updated config:
+```
+rpi-eeprom-config --out pieeprom-${PI_EEPROM_VERSION}-my.bin --config bootconf.txt pieeprom-${PI_EEPROM_VERSION}.bin
+```
+
+As `root` user install the bootrom for flashing the device
+on next boot:
+```
+sudo rpi-eeprom-update -d -f ./pieeprom-${PI_EEPROM_VERSION}-my.bin
+```
+
+The above command will put the files below under `/boot`:
+- `recovery.bin` -- second stage bootloader capable to flash the FW
+- `pieeprom.upd` and `pieeprom.sig` -- prepared FW second stage bootloader to be
+  flashed into EEPROM and its `sha256` checksum
+
+After reboot the new bootrom will be flashed.
+
+# Second stage bootloader
+
+After setting the configuration as described above, the common directory `bcm2711-rpi-4-b` will
+be used on TFTP server to search boot files for all RPi 4 devices.
+
+The boot directory `bcm2711-rpi-4-b/` must contain files used for the second stage
+bootloader (see the 
+[contents of /boot](https://gitlab.apertis.org/pkg/raspi-firmware/-/tree/apertis/v2022dev3/boot)
+from package [raspi-firmware](https://gitlab.apertis.org/pkg/raspi-firmware)).
+
+In addition, the DTB file `bcm2711-rpi-4-b.dtb` from the kernel
+used in Apertis and the `config.txt` boot configuration needs to
+be copied under the same `bcm2711-rpi-4-b/` directory.
+The description of variables for `config.txt` is [available online](https://www.raspberrypi.org/documentation/configuration/config-txt/README.md).
+For Apertis we use singe bootable U-Boot image for RPi3 and RPi4, 64b OS and
+upstream kernel, hence a minimal `config.txt` file should look like this:
+
+```
+kernel=u-boot.bin
+
+# Switch the CPU from ARMv7 into ARMv8 (aarch64) mode
+arm_64bit=1
+
+# Use dtb with upstream names
+upstream_kernel=1
+
+# Enable serial console
+enable_uart=1
+
+# Forced core frequency to allow correct work of UART both
+# for RPi 3 and 4 with DTB from upstream kernel
+[pi3]
+core_freq=250
+
+[pi4]
+core_freq=500
+core_freq_min=500
+```
+
+# U-Boot binary
+
+We are using U-Boot binary from the package [u-boot-rpi](https://gitlab.apertis.org/pkg/u-boot)
+with minimal version `2020.04+dfsg-2co3` which has all features required
+for Apertis. It is possible to use single unified binary `/usr/lib/u-boot/rpi_arm64/u-boot.bin`
+suitable for both RPi3 and RPi4 or use a dedicated U-Boot for each
+board version (need to set the `kernel` variable separately per board in `config.txt` file).
+
+The `u-boot.bin` should be placed alongside the other boot files, so the minimal files subset
+in the boot directory should contain the following files:
+```
+bcm2711-rpi-4-b.dtb
+bootcode.bin
+config.txt
+fixup4.dat
+start4.elf
+u-boot.bin
+```
+
+# OS Boot
+
+With the setup above the U-Boot is responsible for loading kernel and initramfs
+according the configuration provided by `extlinux/extlinux.conf` or
+boot commands for U-Boot set in LAVA job.
+
+For health check the same [network boot configuration](https://gitlab.apertis.org/tests/apertis-test-cases/-/blob/apertis/v2022dev3/lava/profiles.yaml#L165-170)
+may be used as we use for Renesas boards with minimal kernel v5.10.x.