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/uniphier_v7_defconfig b/configs/uniphier_v7_defconfig
index 87eb939..0bedfe1 100644
--- a/configs/uniphier_v7_defconfig
+++ b/configs/uniphier_v7_defconfig
@@ -39,6 +39,8 @@
 # CONFIG_SPL_DOS_PARTITION is not set
 # CONFIG_SPL_EFI_PARTITION is not set
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_USE_BOOTFILE=y
+CONFIG_BOOTFILE="zImage"
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_GPIO_UNIPHIER=y
 CONFIG_MISC=y