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/pengwyn_defconfig b/configs/pengwyn_defconfig
index 5dcb46b..4c46f1e 100644
--- a/configs/pengwyn_defconfig
+++ b/configs/pengwyn_defconfig
@@ -45,6 +45,7 @@
 CONFIG_CMD_EXT4_WRITE=y
 CONFIG_CMD_FAT=y
 CONFIG_CMD_FS_GENERIC=y
+CONFIG_CMD_DIAG=y
 CONFIG_ISO_PARTITION=y
 CONFIG_EFI_PARTITION=y
 CONFIG_MMC_OMAP_HS=y