scripts: Add a script for building and booting QEMU

It is handy to be able to quickly build and boot a QEMU image for a
particular architecture and distro.

Add a script for this purpose. It supports only arm and x86 at present.
For distros it only supports Ubuntu. Both 32- and 64-bit builds are
supported.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/board/emulation/index.rst

@@ -8,6 +8,7 @@ Emulation
 
    acpi
    blkdev
+   script
    qemu-arm
    qemu-mips
    qemu-ppce500

doc/board/emulation/script.rst

@@ -0,0 +1,61 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Script for building and running
+===============================
+
+You may find the script `scripts/build-qemu.sh` helpful for building and testing
+U-Boot on QEMU.
+
+If uses a environment variables to control how it works:
+
+ubdir
+    base directory for building U-Boot, with each board being in its own
+    subdirectory
+
+imagedir
+    directory containing OS images, containin a subdirectory for each distro
+    type (e.g. ubuntu/
+
+Once configured, you can build and run QEMU for arm64 like this::
+
+    scripts/build-qemu.sh -rsw
+
+No support is currently included for specifying a root disk, so this script can
+only be used to start installers.
+
+Options
+~~~~~~~
+
+Options are available to control the script:
+
+-a <arch>
+    Select architecture (default arm, x86)
+
+-B
+    Don't build; assume a build exists
+
+-k
+    Use kvm - kernel-based Virtual Machine. By default QEMU uses its own
+    emulator
+
+-o <os>
+    Run an Operating System. For now this only supports 'ubuntu'. The name of
+    the OS file must remain unchanged from its standard name on the Ubuntu
+    website.
+
+-r
+    Run QEMU with the image (by default this is not done)
+
+-R
+    Select OS release (e.g. 24.04).
+
+-s
+    Use serial only (no display)
+
+-w
+    Use word version (32-bit). By default, 64-bit is used
+
+.. note::
+
+    Note: For now this is a shell script, but if it expands it might be better
+    as Python, accepting the slower startup.