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/walnut_defconfig b/configs/walnut_defconfig
index 6880ae8..1e660ab 100644
--- a/configs/walnut_defconfig
+++ b/configs/walnut_defconfig
@@ -15,6 +15,7 @@
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y
 CONFIG_CMD_SNTP=y
+CONFIG_CMD_DIAG=y
 # CONFIG_MMC is not set
 CONFIG_MTD_NOR_FLASH=y
 CONFIG_SYS_NS16550=y