video: Add commands to list and change fonts

Add a new 'font' command which allows the fonts to be listed as well as
selecting a different font and size.

Allow the test to run on sandbox, where multiple font/size combinations
are supported, as well as sandbox_flattree, where they are not.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 9022101..c601d35 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -48,6 +48,7 @@
    cmd/fatinfo
    cmd/fatload
    cmd/fdt
+   cmd/font
    cmd/for
    cmd/gpio
    cmd/load