Remove/update old generic-board documentation and warning

Remove the warning from the Makefile, since boards that do not use generic
board will no longer build. Also update documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Andreas Bießmann <andreas@biessmann.org>
diff --git a/board/sandbox/README.sandbox b/board/sandbox/README.sandbox
index fa1842b..9fe3bf1 100644
--- a/board/sandbox/README.sandbox
+++ b/board/sandbox/README.sandbox
@@ -186,8 +186,7 @@
 A wide range of commands is implemented. Filesystems which use a block
 device are supported.
 
-Also sandbox uses generic board (CONFIG_SYS_GENERIC_BOARD) and supports
-driver model (CONFIG_DM) and associated commands.
+Also sandbox supports driver model (CONFIG_DM) and associated commands.
 
 
 Linux RAW Networking Bridge