TBB: use SHA256 to generate the certificate signatures

This patch replaces SHA1 by SHA256 in the 'cert_create' tool, so
certificate signatures are generated according to the NSA Suite B
cryptographic algorithm requirements.

Documentation updated accordingly.

Change-Id: I7be79e6b2b62dac8dc78a4f4f5006e37686bccf6
diff --git a/tools/cert_create/src/cert.c b/tools/cert_create/src/cert.c
index 9705643..22fe3d5 100644
--- a/tools/cert_create/src/cert.c
+++ b/tools/cert_create/src/cert.c
@@ -170,7 +170,7 @@
 	}
 
 	/* Sign the certificate with the issuer key */
-	if (!X509_sign(x, ikey, EVP_sha1())) {
+	if (!X509_sign(x, ikey, EVP_sha256())) {
 		ERR_print_errors_fp(stdout);
 		return 0;
 	}