Add sample encryption keys

This adds both RSA (pub + private) keys and a base64 encoded AES-128
secret key, which should be used as sample keys in encrypted image tests.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/enc-aes128kw.b64 b/enc-aes128kw.b64
new file mode 100644
index 0000000..ed2c9fb
--- /dev/null
+++ b/enc-aes128kw.b64
@@ -0,0 +1 @@
+0VoElcTCqP8weM5Jtfyy3Q==