Convert CONFIG_CMD_DIAG to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_DIAG

Signed-off-by: Simon Glass <sjg@chromium.org>
[trini: imply CMD_DIAG on some keymile configs]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 5451398..334e531 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -972,6 +972,14 @@
 	  for some PowerPC processors. For details please see the
 	  docuemntation in doc/README.beddbug
 
+config CMD_DIAG
+	bool "diag - Board diagnostics"
+	help
+	  This command provides access to board diagnostic tests. These are
+	  called Power-on Self Tests (POST). The command allows listing of
+	  available tests and running either all the tests, or specific tests
+	  identified by name.
+
 endmenu
 
 config CMD_UBI