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/T1024RDB_NAND_defconfig b/configs/T1024RDB_NAND_defconfig
index 20ded48..d4dc7bf 100644
--- a/configs/T1024RDB_NAND_defconfig
+++ b/configs/T1024RDB_NAND_defconfig
@@ -61,6 +61,8 @@
 CONFIG_OF_CONTROL=y
 CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_NAND=y
+CONFIG_USE_BOOTFILE=y
+CONFIG_BOOTFILE="uImage"
 CONFIG_DM=y
 CONFIG_FSL_CAAM=y
 CONFIG_SYS_FSL_DDR3=y