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/configs/MiniFAP_defconfig b/configs/MiniFAP_defconfig
index 8c186f7..ee9e99a 100644
--- a/configs/MiniFAP_defconfig
+++ b/configs/MiniFAP_defconfig
@@ -21,6 +21,7 @@
 CONFIG_CMD_BSP=y
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_FAT=y
+CONFIG_CMD_DIAG=y
 CONFIG_MAC_PARTITION=y
 CONFIG_ISO_PARTITION=y
 # CONFIG_MMC is not set