test/py: Add test case for mkimage -o argument

Stress the '-o algo_name' argument of mkimage by expanding the vboot
test.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
[trini: Update scripts/pylint.base]
diff --git a/scripts/pylint.base b/scripts/pylint.base
index 7e25108..e84ae52c 100644
--- a/scripts/pylint.base
+++ b/scripts/pylint.base
@@ -66,7 +66,7 @@
 test_tests_test_ums.py 6.32
 test_tests_test_unknown_cmd.py 5.00
 test_tests_test_ut.py 7.06
-test_tests_test_vboot.py 6.08
+test_tests_test_vboot.py 6.01
 test_tests_vboot_evil.py 8.95
 test_tests_vboot_forge.py 9.22
 test_u_boot_console_base.py 7.08