bootstd: Add a hunter for the extension feature

This needs to run before any bootdev is used, so add a hunter for it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/boot/bootstd_common.h b/test/boot/bootstd_common.h
index 0eb48fa..136a79b 100644
--- a/test/boot/bootstd_common.h
+++ b/test/boot/bootstd_common.h
@@ -20,6 +20,11 @@
 #define TEST_VERSION		"U-Boot v2022.04-local2"
 #define TEST_VERNUM		0x00010002
 
+enum {
+	MAX_HUNTER	= 8,
+	MMC_HUNTER	= 3,	/* ID of MMC hunter */
+};
+
 struct unit_test_state;
 
 /**