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/py/tests/fit_util.py b/test/py/tests/fit_util.py
index fcec4c4..79718d4 100644
--- a/test/py/tests/fit_util.py
+++ b/test/py/tests/fit_util.py
@@ -36,7 +36,7 @@
         print(base_its % params, file=outf)
     return its
 
-def make_fit(cons, mkimage, base_its, params, basename='test.fit'):
+def make_fit(cons, mkimage, base_its, params, basename='test.fit', base_fdt=None):
     """Make a sample .fit file ready for loading
 
     This creates a .its script with the selected parameters and uses mkimage to
@@ -55,6 +55,9 @@
     fit = make_fname(cons, basename)
     its = make_its(cons, base_its, params)
     util.run_and_log(cons, [mkimage, '-f', its, fit])
+    if base_fdt:
+        with open(make_fname(cons, 'u-boot.dts'), 'w') as fd:
+            fd.write(base_fdt)
     return fit
 
 def make_kernel(cons, basename, text):