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>
diff --git a/doc/develop/bootstd/overview.rst b/doc/develop/bootstd/overview.rst
index de15fa4..20c5d56 100644
--- a/doc/develop/bootstd/overview.rst
+++ b/doc/develop/bootstd/overview.rst
@@ -425,6 +425,7 @@
    - 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