FWU: Add FWU support to cert_create tool

Firmware Update requires an X509v3 certificate which contains
hashes for SCP_BL2U, BL2U and NS_BL2U images as extensions.

This patch extends the Chain of Trust definition in the
'cert_create' tool to include the Firmware Update certificate
and the required extensions (including command line options).
A new field in the extension structure will be used to indicate
that the extension is optional. In the case of an image hash
extension, this field will tell the tool that the hash should
be included in the certificate, but filled with zeros.

Change-Id: I1f77a66b018826b71745910771f38d9cf6050388

tools/cert_create/include/ext.h

@@ -72,6 +72,8 @@ typedef struct ext_s {
 	X509V3_EXT_METHOD method; /* This field may be used to define a custom
 				   * function to print the contents of the
 				   * extension */
+
+	int optional;	/* This field may be used optionally to exclude an image */
 } ext_t;
 
 enum {