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/ext.c b/tools/cert_create/src/ext.c
index 14aef66..3f56edb 100644
--- a/tools/cert_create/src/ext.c
+++ b/tools/cert_create/src/ext.c
@@ -69,6 +69,7 @@
  */
 int ext_init(void)
 {
+	cmd_opt_t cmd_opt;
 	ext_t *ext;
 	X509V3_EXT_METHOD *m;
 	int nid, ret;
@@ -78,10 +79,12 @@
 		ext = &extensions[i];
 		/* Register command line option */
 		if (ext->opt) {
-			if (cmd_opt_add(ext->opt, required_argument,
-					CMD_OPT_EXT)) {
-				return 1;
-			}
+			cmd_opt.long_opt.name = ext->opt;
+			cmd_opt.long_opt.has_arg = required_argument;
+			cmd_opt.long_opt.flag = NULL;
+			cmd_opt.long_opt.val = CMD_OPT_EXT;
+			cmd_opt.help_msg = ext->help_msg;
+			cmd_opt_add(&cmd_opt);
 		}
 		/* Register the extension OID in OpenSSL */
 		if (ext->oid == NULL) {