sandbox: Add an explanation of the sandbox variants

There are quite a few builds of sandbox now. Add information about these
to the README.

Signed-off-by: Simon Glass <sjg@chromium.org>

board/sandbox/README.sandbox

@@ -193,6 +193,30 @@ device are supported.
 Also sandbox supports driver model (CONFIG_DM) and associated commands.
 
 
+Sandbox Variants
+----------------
+
+There are unfortunately quite a few variants at present:
+
+sandbox - should be used for most tests
+sandbox64 - special build that forces a 64-bit host
+sandbox_flattree - builds with dev_read_...() functions defined as inline.
+    We need this build so that we can test those inline functions, and we
+    cannot build with both the inline functions and the non-inline functions
+    since they are named the same.
+sandbox_noblk - builds without CONFIG_BLK, which means the legacy block
+    drivers are used. We cannot use both the legacy and driver-model block
+    drivers since they implement the same functions
+sandbox_spl - builds sandbox with SPL support, so you can run spl/u-boot-spl
+    and it will start up and then load ./u-boot. It is also possible to
+    run ./u-boot directly.
+
+Of these sandbox_noblk can be removed once CONFIG_BLK is used everwhere, and
+sandbox_spl can probably be removed since it is a superset of sandbox.
+
+Most of the config options should be identical between these variants.
+
+
 Linux RAW Networking Bridge
 ---------------------------