x86: coral: Enable CONFIG_BOOTARGS_SUBST

Enable this option so that the boot-script substitutions of %U works as
expected. With this, it can boot into Chrome OS.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/chromebook_coral_defconfig b/configs/chromebook_coral_defconfig
index 05e6ce6..ab73a0a 100644
--- a/configs/chromebook_coral_defconfig
+++ b/configs/chromebook_coral_defconfig
@@ -29,6 +29,7 @@
 CONFIG_SPL_BOOTSTAGE_RECORD_COUNT=10
 CONFIG_BOOTSTAGE_STASH=y
 CONFIG_USE_BOOTARGS=y
+CONFIG_BOOTARGS_SUBST=y
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_LOGF_FUNC=y
 CONFIG_SPL_LOG=y