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_is1_defconfig b/configs/socfpga_is1_defconfig
index 2124a08..01b7086 100644
--- a/configs/socfpga_is1_defconfig
+++ b/configs/socfpga_is1_defconfig
@@ -38,6 +38,8 @@
 # CONFIG_EFI_PARTITION is not set
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_USE_BOOTFILE=y
+CONFIG_BOOTFILE="zImage"
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL_DM_SEQ_ALIAS=y
 CONFIG_BOOTCOUNT_LIMIT=y