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/axs101_defconfig b/configs/axs101_defconfig
index 5dd323d..ff2e5e4 100644
--- a/configs/axs101_defconfig
+++ b/configs/axs101_defconfig
@@ -32,6 +32,8 @@
 CONFIG_ENV_IS_IN_FAT=y
 CONFIG_ENV_FAT_DEVICE_AND_PART="0:1"
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_USE_BOOTFILE=y
+CONFIG_BOOTFILE="uImage"
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_HSDK_CREG_GPIO=y
 CONFIG_MMC=y