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/socfpga_secu1_defconfig b/configs/socfpga_secu1_defconfig
index f7bdb90..e71b1e1 100644
--- a/configs/socfpga_secu1_defconfig
+++ b/configs/socfpga_secu1_defconfig
@@ -54,6 +54,8 @@
 CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
+CONFIG_USE_BOOTFILE=y
+CONFIG_BOOTFILE="zImage"
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL_DM_SEQ_ALIAS=y
 # CONFIG_SPL_BLK is not set