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/MPC837XERDB_defconfig b/configs/MPC837XERDB_defconfig
index eac5b74..b493252 100644
--- a/configs/MPC837XERDB_defconfig
+++ b/configs/MPC837XERDB_defconfig
@@ -168,6 +168,8 @@
 CONFIG_OF_CONTROL=y
 CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_ADDR=0xFE080000
+CONFIG_USE_BOOTFILE=y
+CONFIG_BOOTFILE="uImage"
 CONFIG_DM=y
 CONFIG_FSL_SATA=y
 CONFIG_SYS_SATA_MAX_DEVICE=2