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/MPC8548CDS_legacy_defconfig b/configs/MPC8548CDS_legacy_defconfig
index 87d1fd7..71e3d5c 100644
--- a/configs/MPC8548CDS_legacy_defconfig
+++ b/configs/MPC8548CDS_legacy_defconfig
@@ -28,6 +28,8 @@
 CONFIG_OF_CONTROL=y
 CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_ADDR=0xFFF60000
+CONFIG_USE_BOOTFILE=y
+CONFIG_BOOTFILE="8548cds/uImage.uboot"
 CONFIG_DM=y
 CONFIG_DDR_ECC=y
 CONFIG_ECC_INIT_VIA_DDRCONTROLLER=y