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/devkit3250_defconfig b/configs/devkit3250_defconfig
index af17900..2210fab 100644
--- a/configs/devkit3250_defconfig
+++ b/configs/devkit3250_defconfig
@@ -41,6 +41,8 @@
 CONFIG_CMD_JFFS2=y
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_USE_BOOTFILE=y
+CONFIG_BOOTFILE="uImage"
 CONFIG_VERSION_VARIABLE=y
 CONFIG_DMA_LPC32XX=y
 CONFIG_LPC32XX_GPIO=y