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/legoev3_defconfig b/configs/legoev3_defconfig
index 510c4c5..86ff6a1 100644
--- a/configs/legoev3_defconfig
+++ b/configs/legoev3_defconfig
@@ -22,6 +22,7 @@
 CONFIG_CMD_PING=y
 CONFIG_CMD_EXT4=y
 CONFIG_CMD_FAT=y
+CONFIG_CMD_DIAG=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_SYS_NS16550=y