vbe: Add a test for VBE device tree fixups

When a FIT includes some OS requests, U-Boot should process these and add
the requested info to corresponding subnodes of the /chosen node. Add a
pytest for this, which sets up the FIT, runs bootm and then uses a C
unit test to check that everything looks OK.

The test needs to run on sandbox_flattree since we don't support
device tree fixups on sandbox (live tree) yet. So enable BOOTMETH_VBE and
disable bootflow_system(), since EFI is not supported on
sandbox_flattree.

Add a link to the initial documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_flattree_defconfig b/configs/sandbox_flattree_defconfig
index fdd7b35..9d8da25 100644
--- a/configs/sandbox_flattree_defconfig
+++ b/configs/sandbox_flattree_defconfig
@@ -11,7 +11,6 @@
 CONFIG_FIT=y
 CONFIG_FIT_SIGNATURE=y
 CONFIG_FIT_VERBOSE=y
-# CONFIG_BOOTMETH_VBE is not set
 CONFIG_BOOTSTAGE=y
 CONFIG_BOOTSTAGE_REPORT=y
 CONFIG_BOOTSTAGE_FDT=y
@@ -84,6 +83,7 @@
 CONFIG_SYSCON=y
 CONFIG_DEVRES=y
 CONFIG_DEBUG_DEVRES=y
+CONFIG_OFNODE_MULTI_TREE=y
 CONFIG_ADC=y
 CONFIG_ADC_SANDBOX=y
 CONFIG_AXI=y