Support for signing with multiple signature files, key sizes.

If we do a key-rotation in the future, we'll want to sign updates with
two keys. This CL changes the delta generator in a
backwards-compatible way to take multiple key lengths and signature
files: On a command line where one could be given before, now multiple
may be given by colon-delimiting them.

Also, adds two unittests to show that old and new clients can
successfully verify a payload when it's signed by old and new keys.

BUG=chromium-os:19873
TEST=unittests; tested on device

Change-Id: I2063095773a5c71c32704c30b12d6eab2a5f3b80
Reviewed-on: http://gerrit.chromium.org/gerrit/6999
Reviewed-by: Andrew de los Reyes <adlr@chromium.org>
Tested-by: Andrew de los Reyes <adlr@chromium.org>
diff --git a/unittest_key2.pem b/unittest_key2.pem
new file mode 100644
index 0000000..d1f9a78
--- /dev/null
+++ b/unittest_key2.pem
@@ -0,0 +1,27 @@
+-----BEGIN RSA PRIVATE KEY-----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+x6CkWyiyd94xIq5VbnXhvL3a4d4o6OxMWdG5aSsCgYEAo44z1afIzP7WkdzkPIZt
+l/8linR1A1BymBccqsHPN9dIyLP9X3puEc2u6uuH5CXtoLgSZmENXF577L38h0zz
+s34gebf4/RqEUMOj97OAMfxgz+rgs4yO19DEINCYAzPufJjsHEFdTAVFXn5Xl+wg
+QGRwp1ir1Uv64yffjYC9ls0CgYEAjvIxpiKniPNvwUYypmDgt5uyKetvCpaaabzQ
++YpOQJep+wuRYFfCpZotkDf0SHGoR8wnd23GYpIilvPvgyZfp9HuW2n2nhrWROnl
+Cd63IDUwxeOcni7+XA71mwb7HLMC3Jws2geQc8DPZAdIww3P0eT2QYGBcobmI8jO
+akuEYXMCgYAm79Kb/r+3Hew5oAS1Whw70DskVlOutSgNsDPfW9MtDcnETBcGep7A
+1jCL5jjdUYRonimVMFjh1K+UFzV/DQHkgNzjxz9Inbh02y67vL2X836dS9esOcbx
+uZhf+8rL+GnSNqYDqCEuP7qCIloDhguJq9NKyTB4yc59qIkY2zPAzQ==
+-----END RSA PRIVATE KEY-----