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/mx28evk_spi_defconfig b/configs/mx28evk_spi_defconfig
index 6cb21b9..1bac585 100644
--- a/configs/mx28evk_spi_defconfig
+++ b/configs/mx28evk_spi_defconfig
@@ -40,6 +40,8 @@
 CONFIG_CMD_UBI=y
 CONFIG_ENV_OVERWRITE=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_USE_BOOTFILE=y
+CONFIG_BOOTFILE="uImage"
 CONFIG_VERSION_VARIABLE=y
 CONFIG_MXS_GPIO=y
 CONFIG_MMC_MXS=y