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/xilinx-ppc440-generic_defconfig b/configs/xilinx-ppc440-generic_defconfig
index 723da69..c9a1e2b 100644
--- a/configs/xilinx-ppc440-generic_defconfig
+++ b/configs/xilinx-ppc440-generic_defconfig
@@ -17,6 +17,7 @@
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_PING=y
 CONFIG_CMD_CACHE=y
+CONFIG_CMD_DIAG=y
 CONFIG_OF_EMBED=y
 CONFIG_NETCONSOLE=y
 # CONFIG_MMC is not set