Convert CONFIG_BOOTFILE to Kconfig

This converts the following to Kconfig:
   CONFIG_BOOTFILE

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/omapl138_lcdk_defconfig b/configs/omapl138_lcdk_defconfig
index ca1a581..cdba090 100644
--- a/configs/omapl138_lcdk_defconfig
+++ b/configs/omapl138_lcdk_defconfig
@@ -54,6 +54,8 @@
 CONFIG_OF_CONTROL=y
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_USE_BOOTFILE=y
+CONFIG_BOOTFILE="zImage"
 CONFIG_VERSION_VARIABLE=y
 CONFIG_BOOTP_SEND_HOSTNAME=y
 CONFIG_NET_RANDOM_ETHADDR=y