cert: add some test certificates

Adds some test certificates for the regression 81xx cases.

Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/cert/ca.srl b/cert/ca.srl
new file mode 100644
index 0000000..b685c22
--- /dev/null
+++ b/cert/ca.srl
@@ -0,0 +1 @@
+B7C435A31BDC8BEF