Convert CONFIG_CMD_DISPLAY to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_DISPLAY

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/a4m072_defconfig b/configs/a4m072_defconfig
index 25a06cf..ce5edcb 100644
--- a/configs/a4m072_defconfig
+++ b/configs/a4m072_defconfig
@@ -14,6 +14,7 @@
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y
 CONFIG_CMD_SNTP=y
+CONFIG_CMD_DISPLAY=y
 CONFIG_CMD_FAT=y
 CONFIG_MAC_PARTITION=y
 # CONFIG_MMC is not set