doc: Add a description for bootmeth_sandbox

Add documentation for the sandbox bootmeth.

Fix up the compatible string to drop the 'extlinux' part, which is not
relevant to this bootmeth.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>

doc/develop/bootstd/index.rst

@@ -11,3 +11,4 @@ Standard Boot
    pxelinux
    qfw
    cros
+   sandbox

doc/develop/bootstd/overview.rst

@@ -425,6 +425,7 @@ Bootmeth drivers are provided for booting from various media:
    - EFI boot using boot manager
    - Android bootflow (boot image v4)
    - :doc:`QFW <qfw>`: QEMU firmware interface
+   - :doc:`sandbox <sandbox>` used only for testing
 
 Each driver is controlled by a Kconfig option. If no bootmeth driver is
 selected by a compatible string in the devicetree, all available bootmeth

doc/develop/bootstd/sandbox.rst

@@ -0,0 +1,17 @@
+.. SPDX-License-Identifier: GPL-2.0+:
+
+Sandbox Bootmeth
+================
+
+The sandbox bootmeth is only used for testing. It does not provide any facility
+for booting an OS. While sandbox can do all the processing before the actual
+boot, it is not connected in this bootmeth.
+
+When invoked on a bootdev, this bootmeth pretends to find a bootflow and creates
+the associated structure.
+
+When the bootflow is booted, the bootmeth returns `-ENOTSUPP` indicating that it
+is not supported.
+
+The compatible string "u-boot,sandbox-bootmeth" is used for the driver. It is present
+if `CONFIG_BOOTMETH_SANDBOX` is enabled.