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/include/cmd_opt.h b/tools/cert_create/include/cmd_opt.h
index ca48d7c..389aa23 100644
--- a/tools/cert_create/include/cmd_opt.h
+++ b/tools/cert_create/include/cmd_opt.h
@@ -42,9 +42,16 @@
 	CMD_OPT_EXT
 };
 
+/* Structure to define a command line option */
+typedef struct cmd_opt_s {
+	struct option long_opt;
+	const char *help_msg;
+} cmd_opt_t;
+
 /* Exported API*/
-int cmd_opt_add(const char *name, int has_arg, int val);
+void cmd_opt_add(const cmd_opt_t *cmd_opt);
 const struct option *cmd_opt_get_array(void);
 const char *cmd_opt_get_name(int idx);
+const char *cmd_opt_get_help_msg(int idx);
 
 #endif /* CMD_OPT_H_ */