Add support for ECDSA signatures

This adds support for key version 5 which is an EC key using the NIST
P-256 curve parameters. OTAs may be signed with these keys using the
ECDSA signature algorithm with SHA-256.

Change-Id: Id88672a3deb70681c78d5ea0d739e10f839e4567
diff --git a/testdata/testkey_ecdsa.x509.pem b/testdata/testkey_ecdsa.x509.pem
new file mode 100644
index 0000000..b122836
--- /dev/null
+++ b/testdata/testkey_ecdsa.x509.pem
@@ -0,0 +1,10 @@
+-----BEGIN CERTIFICATE-----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=
+-----END CERTIFICATE-----