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/chromebook_samus_tpl_defconfig b/configs/chromebook_samus_tpl_defconfig
index 6839d8c..ef1d770 100644
--- a/configs/chromebook_samus_tpl_defconfig
+++ b/configs/chromebook_samus_tpl_defconfig
@@ -71,6 +71,8 @@
 # CONFIG_SPL_EFI_PARTITION is not set
 CONFIG_ENV_OVERWRITE=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_USE_BOOTFILE=y
+CONFIG_BOOTFILE="bzImage"
 # CONFIG_NET is not set
 CONFIG_REGMAP=y
 CONFIG_SYSCON=y