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/tb100_defconfig b/configs/tb100_defconfig
index 94ed3fb..0adf050 100644
--- a/configs/tb100_defconfig
+++ b/configs/tb100_defconfig
@@ -16,6 +16,8 @@
 CONFIG_OF_CONTROL=y
 CONFIG_OF_EMBED=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_USE_BOOTFILE=y
+CONFIG_BOOTFILE="uImage"
 CONFIG_PHY_GIGE=y
 CONFIG_ETH_DESIGNWARE=y
 CONFIG_DM_SERIAL=y