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/iot_devkit_defconfig b/configs/iot_devkit_defconfig
index 431491e..14a30e3 100644
--- a/configs/iot_devkit_defconfig
+++ b/configs/iot_devkit_defconfig
@@ -27,6 +27,8 @@
 CONFIG_ENV_IS_IN_FAT=y
 CONFIG_ENV_FAT_DEVICE_AND_PART="0:1"
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_USE_BOOTFILE=y
+CONFIG_BOOTFILE="app.bin"
 # CONFIG_NET is not set
 CONFIG_MMC=y
 CONFIG_MMC_DW=y