bootstd: Support running bootdev hunters

Add a way to run a bootdev hunter to find bootdevs of a certain type. Add
this to the 'bootdev hunt' command. Test for this are added in a later
patch, since a useful test needs some hunters to work with.

Signed-off-by: Simon Glass <sjg@chromium.org>

test/boot/bootdev.c

@@ -237,6 +237,9 @@ static int bootdev_test_hunter(struct unit_test_state *uts)
 	ut_assert_nextline("(total hunters: 0)");
 	ut_assert_console_end();
 
+	ut_assertok(bootdev_hunt("mmc1", false));
+	ut_assert_console_end();
+
 	return 0;
 }
 BOOTSTD_TEST(bootdev_test_hunter, UT_TESTF_DM | UT_TESTF_SCAN_FDT);