cert_create: update help message

The help message printed by the cert_create tool using the command
line option -h (or --help) does not correctly list all the available
command line options.

This patch reworks the print_help() function to print the help
messages in a data driven approach. For each command line option
registered, an optional help message can be specified, which will
be printed by print_help().

Help messages for the TBBR options (certificates, keys and images)
are also provided.

Fix a small bug in the short options string passed to getopt_long:
the ':' was missing in the '-a' option (this option must take an
argument).

Fixes ARM-software/tf-issues#337

Change-Id: I9d08c2dfd349022808fcc884724f677eefdc1452
diff --git a/tools/cert_create/src/cert.c b/tools/cert_create/src/cert.c
index bf52645..a559832 100644
--- a/tools/cert_create/src/cert.c
+++ b/tools/cert_create/src/cert.c
@@ -183,19 +183,21 @@
 
 int cert_init(void)
 {
+	cmd_opt_t cmd_opt;
 	cert_t *cert;
-	int rc = 0;
 	unsigned int i;
 
 	for (i = 0; i < num_certs; i++) {
 		cert = &certs[i];
-		rc = cmd_opt_add(cert->opt, required_argument, CMD_OPT_CERT);
-		if (rc != 0) {
-			break;
-		}
+		cmd_opt.long_opt.name = cert->opt;
+		cmd_opt.long_opt.has_arg = required_argument;
+		cmd_opt.long_opt.flag = NULL;
+		cmd_opt.long_opt.val = CMD_OPT_CERT;
+		cmd_opt.help_msg = cert->help_msg;
+		cmd_opt_add(&cmd_opt);
 	}
 
-	return rc;
+	return 0;
 }
 
 cert_t *cert_get_by_opt(const char *opt)