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/test/boot/bootflow.c b/test/boot/bootflow.c
index 8530523..1e8ea75 100644
--- a/test/boot/bootflow.c
+++ b/test/boot/bootflow.c
@@ -329,6 +329,8 @@
 {
 	struct udevice *dev;
 
+	if (!IS_ENABLED(CONFIG_CMD_BOOTEFI_BOOTMGR))
+		return 0;
 	ut_assertok(uclass_get_device_by_name(UCLASS_BOOTMETH, "efi_mgr",
 					      &dev));
 	sandbox_set_fake_efi_mgr_dev(dev, true);