docs/system: document an example booting the versatilepb machine

There is a bit more out there including Aurelien's excellent write up
and older Debian images here:

  https://www.aurel32.net/info/debian_arm_qemu.php
  https://people.debian.org/~aurel32/qemu/armel/

However the web is transitory and git is forever so lets add something
to the fine manual.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Anders Roxell <anders.roxell@linaro.org>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Message-Id: <20210202134001.25738-16-alex.bennee@linaro.org>

docs/system/arm/versatile.rst

@@ -27,3 +27,37 @@ The Arm Versatile baseboard is emulated with the following devices:
    devices.
 
 -  PL181 MultiMedia Card Interface with SD card.
+
+Booting a Linux kernel
+----------------------
+
+Building a current Linux kernel with ``versatile_defconfig`` should be
+enough to get something running. Nowadays an out-of-tree build is
+recommended (and also useful if you build a lot of different targets).
+In the following example $BLD points to the build directory and $SRC
+points to the root of the Linux source tree. You can drop $SRC if you
+are running from there.
+
+.. code-block:: bash
+
+  $ make O=$BLD -C $SRC ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- versatile_defconfig
+  $ make O=$BLD -C $SRC ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf-
+
+You may want to enable some additional modules if you want to boot
+something from the SCSI interface::
+
+  CONFIG_PCI=y
+  CONFIG_PCI_VERSATILE=y
+  CONFIG_SCSI=y
+  CONFIG_SCSI_SYM53C8XX_2=y
+
+You can then boot with a command line like:
+
+.. code-block:: bash
+
+  $ qemu-system-arm -machine type=versatilepb \
+      -serial mon:stdio \
+      -drive if=scsi,driver=file,filename=debian-buster-armel-rootfs.ext4 \
+      -kernel zImage \
+      -dtb versatile-pb.dtb  \
+      -append "console=ttyAMA0 ro root=/dev/sda"